Pressure-induced superconductivity in a ferromagnet UGe₂

Abstract

We confirmed bulk-superconductivity of a ferromagnet UGe 2 by the specific heat measurement, together with the measurements of the electrical resistivity and ac susceptibility, in a pressure range from p = 1.0 to 1.5 GPa, where the Curie temperature T C (= 22-36 K) is still high, but another characteristic temperature T * is close to zero. In this pressure range, the heavy fermion state is found to be formed at low temperatures.Cerium and uranium compounds indicate a variety of phenomena including magnetic and quadrupolar ordering, heavy fermion and anisotropic superconductivity [1]. In these compounds, the RKKY interaction and the Kondo effect compete with each other. The former interaction enhances the long-range magnetic order, while the latter effect quenches the magnetic moments of localized f electrons. Most of the cerium and uranium compounds order magnetically, where the former interaction overcomes the latter effect. When the magnetic ordering temperature is low enough or close to zero, the heavy fermion state is formed at low temperatures. The conduction electrons in the heavy fermion state are highly different from bare electrons. They are interacting electrons, moving slowly in the crystal, which correspond to a large effective mass m * or a large electronic specific heat coefficient γ .When pressure p is applied to the cerium compounds with antiferromagnetic ordering such as CeIn 3 and CePd 2 Si 2 , the Néel tempereture T N shifts to lower temperatures, and the magnetic quantum critical point corresponding to the extrapolation T N → 0 is reached at p = p c [2]. Superconductivity appears around p c . Correspondingly, the heavy fermion state is formed as p approaches p c . This seems to be a general feature, although the sample quality is essentially important for the appearance of superconductivity. This is because superconductivity is most likely to be magnetically-mediated or of a non-s-wave type and then the breaking of Cooperpairs is mainly due to impurities and crystal defects.

Magnon exchange mechanism of ferromagnetic superconductivity

“…The existence of the two Fermi surfaces explains the linear dependence of the specific heat at low temperatures as opposed to the exponential decrease of the specific heat in the BCS theory. In the ferromagnetic phase the specific heat constant γ is smaller than in the superconducting one, which closely matches the experiments with U Ge 2 [6]. The onset of superconductivity, in the preset paper, leads to the appearance of complicated Fermi surfaces in the spin up and spin down momentum distribution functions.…”

“…At the pressure where the superconducting transition temperature is a maximum T sc = 0.8K, the ferromagnetic state is still stable with T c = 32K, and the system undergoes a first order metamagnetic transition between two ferromagnetic phases with different ordered moments [5]. The specific heat coefficient γ = C T increases steeply near 11 kbar and retains a large and nearly constant value [6].…”

“…At the pressure where the superconducting transition temperature is a maximum T sc = 0.8K, the ferromagnetic state is still stable with T c = 32K, and the system undergoes a first order metamagnetic transition between two ferromagnetic phases with different ordered moments [5]. The specific heat coefficient γ = C T increases steeply near 11 kbar and retains a large and nearly constant value [6].The ferromagnets ZrZn 2 and U RhGe are superconducting at ambient pressure with superconducting critical temperatures T sc = 0.29K and T sc = 0.25K respectively. ZrZn 2 is ferromagnetic below the Curie temperature T c = 28.5K with low-temperature ordered moment of µ s = 0.17µ B per formula unit, while for U RhGe T c = 9.5K and µ s = 0.42µ B .…”

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

“…Specific heat measurements under pressure show that the electronic coefficient of the specific heat γ (Fig. 15(e)) increases as the pressure is increased towards P c1 [219,220], consistent with an increase in the electron effective mass near this pressure.…”

“…The value of μ x is determined by extrapolating the magnetization to zero from above H x , the critical field for a metamagnetic transition [216]. (e) Coefficient γ of the electronic specific heat for single crystal (open circles) [219] and polycrystalline (filled circles) [220] samples metamagnetic transitions, proving that the transitions become first order [216]. Accordingly, it is likely that the critical pressures in UGe 2 represent QPTs rather than QCPs.…”