Optical spectra of the heavy fermion uniaxial ferromagnetUGe2

Abstract

We report a detailed study of UGe 2 single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low-frequency interband transition and a narrow freecarrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp increase in the low-frequency mass and reduction in scattering rate below the upper ferromagnetic transition T C = 53 K indicating the emergence of a heavy fermion state triggered by the ferromagnetic order. The characteristic changes are exhibited most strongly at an energy scale below 12 meV. They recover their unrenormalized value above T C and for Ͼ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature, T x ϳ 30 K, observed in transport and thermodynamic experiments. DOI: 10.1103/PhysRevB.78.172406 PACS number͑s͒: 75.30.Mb, 71.27.ϩa, 78.20.Ϫe The possibility of unconventional superconductivity mediated by ferromagnetic fluctuations has long been a subject of theoretical speculation. 1,2 Interest in this subject has been recently piqued with the discovery of superconductivity coexisting with the ferromagnetic state of UGe 2 under pressure.3 UGe 2 is a strongly anisotropic uniaxial ferromagnet with partially filled 5f electron states. Due to correlations and conduction band-5f hybridization, carrier masses are found to be strongly enhanced 4 ͓͑10-25͒m 0 ͔ although specific-heat coefficients still fall an order of magnitude short of the largest values found in antiferromagnetic uranium-based heavy fermion ͑HF͒ compounds. UGe 2 exhibits a Curie temperature that strongly decreases with increasing pressure from about 53 K at ambient pressure to full suppression around 16 kbar. Superconductivity exists in a pressure region from 10 to 16 kbar, just below the complete suppression of ferromagnetism.3 Although superconductivity and ferromagnetism are usually found to be antagonistic phenomena, the observation fits within the now common scenario of finding superconductivity near the zero-temperature termination of a magnetic phase. In this sense it seemed quite natural to associate the superconductivity with being mediated by the magnetic fluctuations that diverge at a quantum critical point ͑QCP͒ perhaps as in the case of pressure driven superconductivity in the antiferromagnetic HF superconductors. However, the paramagnetic to ferromagnetic transition is strongly first order and is not associated with a peak in the effective electronic mass or superconducting transition temperature. It therefore appears that superconductivity is not directly related to the quantum phase transition connecting the ferromagnetic and paramagnetic states. 5,6 In addition to the main ferromagnetic transition, there appears to be an additional weak first-order transition of more enigmatic origin at lower temperatures ͑for p =0, T x =30 K and for T =0, p x Ϸ 12.5 kbar͒, which has been identified via resistivity, 5,7,8 magnetization, 6 and heat capacity. 7,8 The critical...

Low energy electrodynamics of the Kondo-lattice antiferromagnet CeCu2Ge2

“…As in other recent papers [12][13][14][15] that measure low frequency conductivity in heavy fermion compounds, we find evidence of mass enhancement through a dynamic measurement. At temperatures well above the onset of magnetic order and the coherence temperature, the Drude conductivity is broad and featureless, which is consistent with the strong scattering in the normal state.…”

Electrodynamics of correlated electron materials

“…m * (ω, T ) for ω → 0 is enhanced at low temperatures and falls gradually as a function of increasing frequency. Similar trends of m * (ω, T ) and 1/τ (ω, T ) have been observed in the heavy fermion uniaxial ferromagnet UGe 2 (T Curie = 53 K) (Guritanu et al, 2008), the nearly ferromagnetic metal SrFe 4 Sb 12 (T Curie = 53 K) (Kimura et al, 2006), and the itinerant ferromagnet ZrZn 2 (T Curie = 28 K) (Kimura et al, 2007).…”

“…; TÞ and 1=ð! ; TÞ have been observed in the heavy-fermion uniaxial ferromagnet UGe 2 ðT Curie ¼ 53 KÞ (Guritanu et al, 2008), the nearly ferromagnetic metal SrFe 4 Sb 12 (T Curie ¼ 53 K) (Kimura, et al, 2006), and the itinerant ferromagnet ZrZn 2 (T Curie ¼ 28 K) (Kimura et al, 2007). The far-infrared reflectivity of Co-doped FeSi samples is suppressed and the scattering increases when magnetic order sets in (Mena et al, 2006) (see Fig.…”

“…A violation of the so-called Mott or Ioffe-Regel condition means that ðTÞ exceeds the upper limit fixed by the common understanding of a metal for which the mean free path ' should be larger than the atomic distance d (Ioffe and Regel, 1960; Mott, 1990). Numerous counterexamples, such as alkali-doped fullerenes A 3 C 60 (Gunnarsson and Han, 2000), weakly doped La 2 CuO 4 (Calandra and Gunnarsson, 2002), or some organic conductors, for instance, -ðBEDT-TTFÞ 2 I 3 (Takenaka et al, 2005;Gunnarsson and Vafayi, 2007), are discussed for different reasons. Gunnarsson et al (2003) argued that in cuprates the resistivity saturates at some higher level due to a strong reduction of the kinetic energy caused by correlation effects, while for A 3 C 60 (and probably in a different way for the organics) coupling to phonons becomes important.…”

Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe2

“…Formation of such a narrow SP band should be directly relevant to a large electron mass enhancement and critical spin fluctuations as are inferred from optical conductivity measurements in UGe 2 87 and several other strongly correlated materials including UPd 2 Al 3 101 , UPt 3 101 , Cd 2 Re 2 O 7 102 , MnSi 103 and UBe 13 104 . At low temperature, all of these materials exhibit a narrow peak separated by a hybridization gap from a broad mid-infrared excitation feature in the frequency-dependent optical conductivity, which are associated with a low-spectral-weight, low-energy "heavy fermion" Drude peak and a high-spectral-weight, high-energy interband transition, respectively.…”