Heavy-fermion Compound YbRh2Si2 Research Articles

A Knudsen cell approach for the molecular beam epitaxy of the heavy fermion compound YbRh[formula omitted]Si[formula omitted

Thin films of the heavy fermion compound YbRh2Si2 were grown by molecular beam epitaxy on Ge (001) substrates using effusion cells. As-grown YbRh2Si2 thin films were characterized by a wide range of characterization techniques. X-ray diffraction yields a set of (00l) peaks, demonstrating epitaxial growth along the crystallographic c direction, with a lattice parameter c ranging from 9.84Å–9.95Å. The electrical resistivity shows behavior similar to YbRh2Si2 films grown previously using electron-beam evaporators for Rh and Si. The most stoichiometric sample appears to have the highest quality: It has the highest intensity ratio of the YbRh2Si2 (004) diffraction peak to the Ge (004) peak, the highest R(10 K)/R(2.3 K) ratio, a smallest surface roughness, and only a small density of surface defects.

Temperature-dependent change of the electronic structure in the Kondo lattice system YbRh2Si2

The heavy-fermion behavior in intermetallic compounds manifests itself in a quenching of local magnetic moments by developing Kondo spin-singlet many-body states combined with a drastic increase of the effective mass of conduction electrons, which occurs below the lattice Kondo temperature T K. This behavior is caused by interactions between the strongly localized 4f electrons and itinerant electrons. A controversially discussed question in this context is how the localized electronic states contribute to the Fermi surface upon changing the temperature. One expects that hybridization between the local moments and the itinerant electrons leads to a transition from a small Fermi surface in a non-coherent regime at high temperatures to a large Fermi surface once the coherent Kondo lattice regime is realized below T K. We demonstrate, using hard x-ray angle-resolved photoemission spectroscopy that the electronic structure of the prototypical heavy fermion compound YbRh2Si2 changes with temperature between 100 and 200 K, i.e. far above the Kondo temperature, T K = 25 K, of this system. Our results suggest a transition from a small to a large Fermi surface with decreasing temperature. This result is inconsistent with the prediction of the dynamical mean-field periodic Anderson model and supports the idea of an independent energy scale governing the change of band dispersion.

Spin-flip scattering of critical quasiparticles and the phase diagram ofYbRh2Si2

Several observed transport and thermodynamic properties of the heavy-fermion compound YbRh2Si2 in the quantum critical regime are unusual and suggest that the fermionic quasiparticles are critical, characterized by a scale-dependent diverging effective mass. A theory based on the concept of critical quasiparticles (CQP) scattering off antiferromagnetic spin fluctuations in a strong-coupling regime has been shown to successfully explain the unusual existing data and to predict a number of so far unobserved properties. In this paper, we point out a new feature of a magnetic field-tuned quantum critical point of a heavy-fermion metal: anomalies in the transport and thermodynamic properties caused by the freezing out of spin-flip scattering of critical quasiparticles and the scattering off collective spin excitations. We show that a step-like behavior as a function of magnetic field of e.g. the Hall coefficient and magnetoresistivity results, which accounts quantitatively for the observed behavior of these quantities. That behavior has been described as a crossover line T*(H) in the T - H phase diagram of YbRh2Si2. Whereas some authors have interpreted this observation as signaling the breakdown of Kondo screening and an associated abrupt change of the Fermi surface, our results suggest that the T* line may be quantitatively understood within the picture of robust critical quasiparticles.

Strong-coupling theory of heavy-fermion criticality

We present a theory of the scaling behavior of the thermodynamic, transport and dynamical properties of a three-dimensional metal at an antiferromagnetic critical point. We show how the critical spin fluctuations at the AFM wavevector q=Q induce energy fluctuations at small q, giving rise to a diverging quasiparticle effective mass over the whole Fermi surface. The coupling of the fermionic and bosonic degrees of freedom leads to a self-consistent relation for the effective mass, which has a strong coupling solution in addition to the well-known weak-coupling, spin-density-wave solution. We thereby use the recently-introduced concept of critical quasiparticles, employing a scale-dependent effective mass ratio m*/m and quasiparticle weight factor Z. As a consequence of the diverging effective mass the Landau Fermi liquid interaction is found to diverge in all channels except the critical one, causing important vertex corrections. The ensuing spin fluctuation spectrum obeys omega/T scaling. Our results are in good agreement with experimental data on the heavy fermion compounds YbRh2Si2$ and CeCu(6-x)Au(x) assuming 3D and 2D spin fluctuations, respectively.

Temperature and Magnetic Field Dependent Yb Valence in YbRh2Si2 Observed by X-ray Absorption Spectroscopy

The temperature and magnetic field dependences of Yb valence were observed in the heavy fermion compound YbRh2Si2 by X-ray absorption spectroscopy. The measurements revealed that the Yb valence decreases with decreasing temperature in the range from 200 to 2 K and increases with increasing magnetic field in the range from 0 to 33 T without showing an abrupt change in the Yb valence. The Yb valence is in the range from 2.92 to 2.96 depending on temperature and magnetic field. With respect to the valence being 2.92 at 0 T and 2.93 at 33 T in 2 K, YbRh2Si2 is a valence fluctuation compound and does not reach the integer trivalent state at high magnetic field. These results endorse the conventional knowledge that the valence of Yb is very close to the integer value of 3+, decreases with decreasing temperature, and becomes closer to 3+ with increasing magnetic field.

Verification of the Wiedemann-Franz law in YbRh2Si2 at a quantum critical point.

The thermal conductivity measurements are performed on the heavy-fermion compound YbRh(2)Si(2) down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at B(c)=0.66 T∥c axis. In the limit as T→0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of the unconventional quantum criticality of YbRh(2)Si(2).

Single-crystal growth of YbRh2Si2 and YbIr2Si2

We report on the single-crystal growth of the heavy-fermion compounds YbRh2Si2 and YbIr2Si2 using a high-temperature indium-flux technique. The optimization of the initial composition and the temperature–time profile lead to large (up to 100 mg) and clean (ρ0 ≈ 0.5 µΩ cm) single crystals of YbRh2Si2. Low-temperature resistivity measurements revealed a sample-dependent temperature exponent below 10 K, which for the samples with highest quality deviates from a linear-in-T behaviour. Furthermore, we grew single crystals of the alloy series Yb(Rh1−x Ir x )2Si2 with 0 ≤ x ≤ 0.23 and report the structural details. For pure YbIr2Si2, we establish the formation of two crystallographic modifications, where the magnetic 4f electrons have different physical ground states.

Thermopower Evidence for an Abrupt Fermi Surface Change at the Quantum Critical Point ofYbRh2Si2

We present low-temperature thermopower results, S(T), on the heavy-fermion compound YbRh2Si2 in the vicinity of its field-induced quantum critical point (QCP). At B=0, a logarithmic increase of -S(T)/T between 1 and 0.1 K reveals strong non-Fermi-liquid behavior. A pronounced downturn of -S(T)/T below T{max}=0.1 K and a sign change from negative to positive S(T) values at T{0} approximately 30 mK are observed on the low-field side of the Kondo breakdown crossover line T{*}(B). In the field-induced, heavy Landau-Fermi-liquid regime, S(T)/T assumes constant, negative values below T{LFL}. A pronounced crossover in the -S(B)/T isotherms at T{*}(B) sharpens with decreasing T and seems to evolve toward a steplike function for T-->0. This is attributed to an abrupt change of the Fermi volume upon crossing the unconventional QCP of YbRh2Si2.

High-field ESR study of the Kondo lattice system YbRh2Si2

A multi-frequency electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 has been performed in fields up to ∼ 8T. In this regime the system is close to the breakdown of the heavy-fermion behavior which is confined to temperatures and fields T <T0 ≈25K and B < B* ≈10T, respectively. A well-defined strongly anisotropic ESR mode is observed. We find striking similarities between the properties of this signal and the behavior of the electronic specific heat and the 29Si NMR data which reflect the crossover between different electronic regimes in the Kondo state. These similarities suggest that the observed ESR mode corresponds to a "heavy electron spin resonance", i.e., a joint resonance excitation of the strongly hybridized f- and conduction electron states.

Low-temperature thermopower study of YbRh2Si2

The heavy-fermion compound YbRh2Si2 exhibits an antiferromagnetic (AFM) phase transition at an extremely low temperature of Tn = 70 mK. Upon applying a tiny magnetic field of Bc = 60 mT the AFM ordering is suppressed and the system is driven toward a field-induced quantum critical point (QCP). Here, we present low-temperature thermopower S(T) measurements of high-quality YbRh2Si2 single crystals down to 30 mK. S(T) is found negative with comparably large values in the paramagnetic state. In zero field no Landau-Fermi-liquid (LFL) like behavior is observed within the magnetically ordered phase. However, a sign change from negative to positive appears at lowest temperatures on the magnetic side of the QCP. For higher fields B > Bc a linear extrapolation of S to zero clearly evidences the recovery of LFL regime. The crossover temperature TlFl is sharply determined and coincides perfectly with the one derived from resistivity ρ(T) and specific heat cp(T) investigations.

On the local and itinerant properties of the ESR in YbRh2Si2

Below the Kondo temperature the heavy Fermion compound YbRh2Si2 shows a well defined electron spin resonance (ESR) with local Yb3+ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb3+ in YPd3:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic 174YbRh2Si2 we could exclude unresolved hyperfine contributions to the lineshape.

Crystal Electric Field Excitations in the Non-Fermy Liquid Compound YbRh2Si2

We have calculated the crystalline electric field (CEF) splitting of the energy levels of Yb3+ (4f13) in the clean Yb-based heavy fermion compound YbRh2Si2. The data of inelastic neutron scattering and electron spin resonance measurements in YbRh2Si2, together with relevant structural, thermodynamic, and magnetic properties, were used as input in the calculations of the possible CEF level scheme in this non-Fermi-liquid compound. Two possible sets of the CEF parameters with the Γ6 or Γ7 ground-state symmetry are discussed.