Heavy-fermion formation at the metal-to-insulator transition inGd1−xSrx

M. Heinrich

¹

,

H.-A. Krug von Nidda

²

,

V. Fritsch

³

et al.

Abstract

The perovskite-like transition-metal oxide Gd1−xSrxTiO3 is investigated by measurements of resistivity, specific-heat, and electron paramagnetic resonance (EPR). Approaching the metal-toinsulator transition from the metallic regime (x ≥ 0.2), the Sommerfeld coefficient γ of the specific heat becomes strongly enhanced and the resistivity increases quadratically at low temperatures, which both are fingerprints of strong electronic correlations. The temperature dependence of the dynamic susceptibility, as determined from the Gd 3+ -EPR linewidth, signals the importance of strong spin fluctuations, as observed in heavy-fermion compounds. PACS: 71.30, 71.27, 76.30 The transition-metal oxides RTiO 3 (where R denotes Y 3+ , La 3+ , or some trivalent rare-earth ion) are known as typical Mott-Hubbard insulators with Ti 3+ in a 3d 1 electronic configuration [1]. Doping R 1−x A x TiO 3 with divalent alkali-earth ions A like Ca 2+ or Sr 2+ partially changes the Ti valence to Ti 4+ (3d 0 ) and induces metallic behavior at a certain doping concentration x. The metal-to-insulator transition of La 1−x Sr x TiO 3 and Y 1−x Ca x TiO 3 has been investigated in detail [2,3]. A critical increase of the Sommerfeld coefficient γ of the specific heat approaching the metal-to-insulator transition has been observed in both compounds with γ ≈ 25 mJ/(mole K 2 ) in Y 0.6 Ca 0.4 TiO 3 , indicating an enhancement of the effective electron mass. Heavy-fermion formation has been observed in a number of transitionmetal oxides, with Sr 1−x Ca x RuO 3 [4] and LiV 2 O 4 [5] being the most prominent examples. In the latter two compounds the temperature dependence of the dynamic susceptibility, as probed by the spin-lattice relaxation time in NMR experiments, played a key role in identifying the heavy-fermion behavior. In this letter we investigate the dynamic susceptibility utilizing Gd EPR and provide experimental evidence of heavy-fermion behavior in Gd 1−x Sr x TiO 3 .Ceramic samples of Gd 1−x Sr x TiO 3 have been calcinated from TiO 2 , SrCO 3 , Gd 2 O 3 and Ti 2 O 3 powders of high purity (better than 3N) and pressed into pellets. These pellets have been annealed at 1473 K under N 2 atmosphere for 15 hours and finally arc-melted under argon atmosphere. For Sr concentrations x ≤ 0.7 X-ray powder diffraction revealed the proper orthorhombically distorted GdFeO 3 perovskite structure [6]. For x > 0.7 the system attains the cubic perovskite structure like pure SrTiO 3 . The volume of the unit cell remains nearly constant at about 235Å 3 in the whole concentration range. Susceptibility measurements were performed with a commercial SQUID magnetometer (Quantum Design) in a temperature range 4.2 ≤ T ≤ 400 K. Pure GdTiO 3 shows a ferrimagnetic susceptibility, which follows a Curie-Weiss law χ ∝ (T + Θ) −1 with Θ = 10 K at high temperatures T > 100 K, and exhibits an ordering temperature T C ≈ 28 K. This fits reasonably to the value of 34 K given in literature [7], where the ordered state is reported to show an antiparallel alignment of the ferromagne...

Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3quantum wells

“…We briefly discuss why interfacial intermixing or roughness cannot be responsible for the measured structural distortions. Gd x Sr 1-x TiO 3 alloys remain cubic up to Gd concentrations of x = 0.3 [31]. Therefore very large concentrations of Gd intermixing would need to be present to induce an orthorhombic distortion.…”

Growth and properties of GdTiO3 films prepared by hybrid molecular beam epitaxy

“…GdTiO 3 can be hole-doped by substituting divalent ions, such as Sr, on the Gd-site, or by oxygen excess. At large doping, GdTiO 3 becomes metallic [3,5]. The nature of the band gap in the Mott insulating state has been explored in optical studies, which show a mid-infrared absorption attributed to 3d-3d transitions across the Mott-Hubbard gap, in addition to the O 2p-Ti 3d charge transfer excitations [6,7].…”

Magnetoelastic coupling inRTiO3(R=La,NdKomarek1, Roth2, Cwik3 et al. 2007Phys. Rev. B129111540View full textAdd to dashboardCite

“…In the first class, the RE magnetic moments order ferromagnetically and couple antiparallel to the ferromagnetic component of the Ti sites (F z ). In the second class (RE=Tb, Dy and Ho) the magnetic moments at the RE sites have both ferromagnetic and antiferromagnetic components in the a, b plane 16,28,29,30,31 . Among the heavier RETiO 3 materials only GdTiO 3 has been studied in this work, it exhibits ferromagnetic Gdmoment ordering (first group).…”