Crystalline electric field excitations of the non-Fermi-liquid

Stockert, O.; Koza, Michael Marek; Ferstl, J.; Murani, A.P.; Geibel, C.; Steglich, F.

doi:10.1016/j.physb.2006.01.059

Cited by 54 publications

(57 citation statements)

References 2 publications

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“…[5] Transport and thermodynamic properties are consistent with a single-ion Kondo temperature T K = 25 K (associated with the crystallineelectric-field-derived doublet ground state [6]). …”

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“…Several highly correlated metals exhibit so-called nonFermi liquid (NFL), i.e., strong deviations from a renormalized LFL behavior when T → 0 K. [1] The system YbRh 2 Si 2 studied in this paper is one of a few clean stoichiometric HF metals with pronounced NFL behavior at ambient pressure which is related to both antiferromagnetic (AF) as well as ferromagnetic quantum critical spin fluctuations in close proximity to an AF quantum critical point (QCP). [2, 3, 4] Those NFL effects manifest as a divergence of the 4f -derived increment to the specific heat ∆C/T ∝ − ln T and in the electrical resistivity ρ(T ) showing a power law exponent close to 1 in a temperature range substantially larger than one decade and extending up to T ≃ 10 K.[5] Transport and thermodynamic properties are consistent with a single-ion Kondo temperature T K = 25 K (associated with the crystallineelectric-field-derived doublet ground state [6]). …”

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Optical observation of non-Fermi-liquid behavior in the heavy fermion state ofYbRh2Si2

et al. 2006

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We report far-infrared optical properties of YbRh2Si2 for photon energies down to 2 meV and temperatures 0.4 -300 K. In the coherent heavy quasiparticle state, a linear dependence of the lowenergy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh2Si2 which is due to its close vicinity to an antiferromagnetic quantum critical point.PACS numbers: 71.10. Hf, 71.27.+a, The investigation of 4f -containing metals by farinfrared optical spectroscopy provides valuable insight into the nature of strong electronic correlations. This in particular holds true for heavy fermion (HF) compounds where at low temperatures a weak 4f -conduction electron (cf -)hybridization generates mass-renormalized quasiparticles with a coherent ground state which is in many HF systems of the Landau Fermi liquid (LFL) type.[1] The quasiparticles influence thermodynamic quantities which are described in terms of a large effective mass m * exceeding the free electron mass m 0 by three orders of magnitude. Furthermore, in typical HF materials, below a single-ion Kondo temperature (T K ), the coherent state is characterized by a dynamical screening of the 4f magnetic moments through the conduction electrons. Several highly correlated metals exhibit so-called nonFermi liquid (NFL), i.e., strong deviations from a renormalized LFL behavior when T → 0 K. [1] The system YbRh 2 Si 2 studied in this paper is one of a few clean stoichiometric HF metals with pronounced NFL behavior at ambient pressure which is related to both antiferromagnetic (AF) as well as ferromagnetic quantum critical spin fluctuations in close proximity to an AF quantum critical point (QCP). [2, 3, 4] Those NFL effects manifest as a divergence of the 4f -derived increment to the specific heat ∆C/T ∝ − ln T and in the electrical resistivity ρ(T ) showing a power law exponent close to 1 in a temperature range substantially larger than one decade and extending up to T ≃ 10 K.[5] Transport and thermodynamic properties are consistent with a single-ion Kondo temperature T K = 25 K (associated with the crystallineelectric-field-derived doublet ground state [6]).The electrodynamical response of HF systems is characterized by an optical conductivity σ(ω) which follows at room temperature the classical Drude model [σ(ω) = N e 2 τ /m * (1 + ω 2 τ 2 ); N : charge carrier density] with frequency independent m * and scattering rate 1/τ . [7] At low temperatures, upon entering the coherent state, large deviations are observed which are caused by many-body effects. Then a narrow, renormalized peak at zero photon energy ω = 0 eV is formed and a socalled hybridization gap appears which is related to the transition between the bonding and antibonding states resulting from the cf -hybridization. [8, 9, 10] The coherent part of the underlying strong electron-electron correlations are treated in an extended Drude model by renormalized and frequency dependent m * (ω)...

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“…[5] Transport and thermodynamic properties are consistent with a single-ion Kondo temperature T K = 25 K (associated with the crystallineelectric-field-derived doublet ground state [6]). …”

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confidence: 99%

Optical observation of non-Fermi-liquid behavior in the heavy fermion state ofYbRh2Si2

et al. 2006

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“…However, the fit of the HF-ESR data requires a significant reduction of ∆ from ∼ 115 K (Ref. 7) to ∼ 50−60 K. Moreover, from neutron scattering results this excitation energy ∆ is much higher (∆ ∼ 200 K) [17]. Therefore, the exponential term in ∆B(T ) is not related to the relaxation via excited states.…”

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Evolution of the Kondo State ofYbRh2Si2Probed by High-Field ESR

et al. 2009

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An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to ∼ 8 T reveals a strongly anisotropic signal below the single ion Kondo temperature TK ∼ 25 K. A remarkable similarity between the T -dependence of the ESR parameters and that of the specific heat and the 29 Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions which are formed below TK and that ESR properties are determined by their field dependent mass and lifetime. The signal anisotropy, otherwise typical for Yb 3+ ions, suggests that, owing to a strong hybridization with conduction electrons at T < TK, the magnetic anisotropy of the 4f states is absorbed in the ESR of heavy quasiparticles. Tuning the Kondo effect on the 4f states with magnetic fields ∼ 2 − 8 T and temperature 2 − 25 K yields a gradual change of the ESR g-factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system. PACS numbers: 71.27.+a, 75.20.Hr, Strong electron-electron (EE) interactions in metals yield a fascinating variety of novel and often interrelated quantum phenomena, such as quantum phase transitions, breakdown of the Landau Fermi-liquid (LFL) state, unconventional superconductivity, etc. (for an overview see, e.g., [1]). In intermetallic compounds where 4f (5f ) magnetic ions (e.g. Yb, Ce, U etc.) build up a regular Kondo lattice, strong EE correlations are established by the coupling of local f -magnetic moments with the conduction electrons (CE). As a consequence, a large effective mass enhancement of the quasiparticles (QP) hallmarks the properties of paramagnetic heavy fermion metals. A competing interaction, the so-called RKKY-interaction between the local f -states via the sea of CE, favors a magnetically ordered ground state.An important realization of a system where the delicate balance between Kondo and RKKY interactions can be investigated is the intermetallic compound YbRh 2 Si 2 where antiferromagnetic order, quantum criticality, heavy fermion-and non-LFL (NFL) behavior can be tuned by a magnetic field B and temperature T [2,3,4,5,6] (Fig. 1). In the parameter domain where these remarkable electronic crossovers take place a strong hybridization of 4f electrons with CE significantly broadens the otherwise atomically sharp f -states. That is why the observation of a narrow electron spin resonance (ESR) signal in the Kondo state of YbRh 2 Si 2 was very surprising [7]. While the reported pronounced anisotropy of the signal is indeed in accordance with an ESR of localized Yb 3+ 4f moments, a non-local picture is suggested by the observation of this signal down to the lowest accessible temperatures of 0.69 K [8] where the single ion Kondo effect is expected to screen the magnetic moments. On the other hand the conduction electron ESR seems also unlikely because in this compound comprising heavy metal elements the spin-orbit (SO) coupling drastically shortens the electron spin lifetime [9].To unravel a controversial nature of this resonance response w...

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“…Inelastic neutron scattering experiments showed that these types of behavior are unrelated to the excited crystalline electric field states [239]. By analogy to CeCu 6−x Au x , these results were taken as evidence for 2D antiferromagnetic critical fluctuations near the QCP [47,236], although it should be pointed out that this conclusion remains an open issue.…”

Section: Yb 2 Fe 12 Pmentioning

confidence: 98%

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

et al. 2010

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Standard models for simple metals and insulators often fail for systems based on elements with unstable d-or f -electron shells, where strong electronic correlations can generate new and unexpected states of matter. Such a scenario can often be induced when a magnetic phase transition is tuned to absolute zero temperature by an external control parameter such as chemical composition, pressure or magnetic field. At the resulting quantum critical point (QCP), emergent phenomena, such as unconventional superconductivity and novel magnetic phases are frequently observed. The temperature and energy dependences of the physical properties are also found to deviate from expectations for a simple Fermi liquid. This "non-Fermi-liquid" (NFL) behavior is commonly manifested as weak power laws and logarithmic divergences in the physical properties at low temperatures and is often found in a V-shaped region near a QCP, which has become the "classic" QCP phase diagram. However, there is also a growing number of materials where the NFL behavior either occurs far away from the QCP, within an ordered phase, or may not be associated with any putative QCP. Thus, after nearly 20 years of research, it remains unknown whether NFL physics is universal, or if a multitude of unique subclasses exist. In this article, we review research that has primarily been carried out in our laboratory on systems that exhibit NFL behavior that does not conform to the "classic" QCP scenario.

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Crystalline electric field excitations of the non-Fermi-liquid

Cited by 54 publications

References 2 publications

Optical observation of non-Fermi-liquid behavior in the heavy fermion state ofYbRh2Si2

Optical observation of non-Fermi-liquid behavior in the heavy fermion state ofYbRh2Si2

Evolution of the Kondo State ofYbRh2Si2Probed by High-Field ESR

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

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