Gap opening with ordering inPrFe4P12studied by local tunneling spectroscopy

Abstract

We present measurements of the local tunneling density of states in the low temperature ordered state of PrFe4P12. The temperature dependencies of the Fermi level density of states, and of the integrated density of states at low bias voltages, show anomalies at T ≃ 6.5K, the onset of multipolar ordering as detected by specific heat and other macroscopic measurements. In the ordered phase, we find a local density of states with a V-shape form, indicating partial gap opening over the Fermi surface. The size of the gap according to the tunneling spectra is about 2 meV.PACS numbers: 71.27. a+,71.30 +h, 73.20.-r Recently, a new family of heavy fermion f-electron intermetallic compounds with the filled skutterudite structure RT 4 X 12 (R is a rare earth, T a transition metal and X a pnictogen) has been found to show many attractive phenomena, most of them appearing at low temperatures. Among others, metal-insulator transition, heavy fermion behavior, and superconductivity have been found in (R=Pr) systems [1,2,3,4,5,6].In particular, in PrFe 4 P 12 , a peculiar phase transition occurs at T A = 6.5 K, with distinct anomalies in both resistivity [7] and specific heat [8]. The intriguing order below T A is widely believed to be associated with orbital degrees of freedom [1]. A recent NMR study [9] has put severe restrictions on the symmetry of the order parameter. According to it, the ordering involves non-magnetic multipoles, which do not break the point symmetry of the crystal at the Pr sites. The scalar order with Γ 1g [10] is currently the most promising candidate to represent the order parameter. On the other hand, according to transport studies [6,7,11], the itinerant electrons are profoundly affected by this phase transition. The multifold increase in the Hall, Seebeck and Nernst coefficients all suggest a drastic decrease in the number of charge carriers [11]. This is compatible with the results of a de Haas van Alphen study [6], which found a small Fermi surface (0.0015 of the Brillouin zone) with a moderate effective mass (∼10m e ) in the ordered state. In higher magnetic fields (strong enough to destroy the ordering), the same study resolved several larger Fermi surfaces with heavier masses up to 81m e [6]. The consequences of the ordering on transport properties recall the case of URu 2 Si 2 [12]. In both cases, the ordering leads to an enhanced lattice thermal conductivity, a large Hall coefficient and anomalously large Nernst and Seebeck coefficients due to an incomplete metal-insulator transition, producing a heavy-Fermion semi-metal. In the case of URu 2 Si 2 , a partial gap opening has been observed using different techniques [13,14].On the other hand, scanning tunneling microscopy and spectroscopy (STM/S) techniques have been widely applied to systems where some kind of energy gap appears in the quasiparticle spectrum. The superconducting gap is certainly one of the most studied kinds of energy gaps viewed using this technique, although different kinds of charge ordered, and semimetallic or semicondu...

Temperature dependent tunneling spectroscopy in the heavy fermion CeRu2Si2and in the antiferromagnet CeRh2Si2

“…The latter opens new perspectives, providing direct information about electronic band structure and correlations. Successful STM measurements have been reported in PrOs 4 Sb 12 [7], PrFe 4 P 12 [8], YbRh 2 Si 2 [9], URu 2 Si 2 [10,11] and in CeRhIn 5 and CeCoIn 5 [12].…”

Electron-phonon superconductivity in the filled skutterudites LaRu4P12, LaRu4As12 , and Tütüncü1, Karaca2, Srivastava3 2017Phys. Rev. B

“…Filled skutterudite pnictides with the general formula LnT 4 X 12 (Ln = lanthanide, T = transition metal, X = pnictogen) have received considerable attention in recent years because of their interesting physical properties at low temperature, such as excellent thermoelectric performance [1][2][3][4], intermediate-valence [5], non-Fermi-liquid behavior [6], magnetic ordering [7][8][9], metal-to-insulator transition [10][11][12][13][14][15][16], semiconducting behavior [17][18][19][20][21][22][23], and heavy-fermion behavior [24][25][26][27][28][29][30][31][32][33][34][35][36]. Also, lanthanum compounds display interesting superconducting behavior at low temperatures [37][38][39][40][41][42][43][44][45][46][47].…”

A new ternary magnetically ordered heavy fermion compound Pr2Rh3Ge: magnetic, electronic and thermodynamic properties

“…First is the quadrupolar Kondo effect (examples being PrInAg 2 [15,16] or PrFe 4 P 12 [17,18]), which is very different from that in Ce compounds, where the HF behaviour is frequently a consequence of the spin Kondo effect on the Ce sites due to strong c-f hybridization and forming the 4f-derived heavy electronic states. It is worth noting that using local tunneling spectr oscopy with a scanning tunneling microscope (STM), it turned out that in the case of PrFe 4 P 12 , the magnitude of decrease in the conductance integral indicates that about onethird of the electrons participating in the tunneling process come from the gapped part of the Fermi surface [19]. The rest, i.e.…”