[Paper Review] Band-Selective Modification of the Magnetic Fluctuations in Sr2RuO4: Study of Substitution Effects

Naoki Kikugawa, C. Bergemann+2 more|arXiv (Cornell University)|Nov 13, 2002

Advanced Condensed Matter Physics4 citations

TL;DR

This study investigates band-selective modification of magnetic fluctuations in the spin-triplet superconductor Sr2RuO4 via La3+ substitution for Sr2+, which electron-dopes the system. Using transport, specific heat, and susceptibility measurements, it demonstrates that approaching a van Hove singularity in the γ-band Fermi surface enhances two-dimensional ferromagnetic spin fluctuations, leading to non-Fermi-liquid behavior near y ≈ 0.20, while Ti substitution instead enhances antiferromagnetic fluctuations via α–β nesting.

ABSTRACT

We report a study of magnetic, thermal, and transport properties of La(3+) substituted Sr2RuO4, performed in order to investigate the effects of additional electron doping in this correlated metal. A gradual enhancement of the electronic part of specific heat and a more drastic increase of the static magnetic susceptibility were observed in Sr(2-y)La(y)RuO(4) with increasing y. Furthermore, the quasi-two-dimensional Fermi-liquid behavior seen in pure Sr2RuO4 breaks down near the critical concentration y ~ 0.20. Combined with a realistic tight-binding model with rigid-band shift of Fermi level, the enhancement of the density of states can be ascribed to the elevation of the Fermi energy toward a van Hove singularity of the thermodynamically dominant γFermi-surface sheet. On approaching the van Hove singularity, the effective nesting-vector of the γband shrinks and further enhances the susceptibility near the wave vector q ~ 0. We attribute the non-Fermi-liquid behavior to two-dimensional ferromagnetic fluctuations with short range correlations at the van Hove singularity. The observed behavior is in sharp contrast to that of Ti(4+) substitution in Sr2RuO4 which enhances antiferromagnetic fluctuations and subsequently induces incommensurate magnetic ordering associated with the nesting between the other Fermi-surface sheets (αand β). We thus establish that substitution of appropriate chemical dopants can band-selectively modify the spin-fluctuation spectrum in the spin-triplet superconductor Sr2RuO4.

Motivation & Objective

• To investigate the effects of electron doping via La3+ substitution on magnetic, thermal, and transport properties in Sr2RuO4.
• To determine whether electron doping selectively enhances spin fluctuations in specific Fermi surface sheets, particularly the γ-band.
• To examine the emergence of non-Fermi-liquid behavior near a quantum critical point associated with a van Hove singularity.
• To contrast the effects of La3+ doping (ferromagnetic fluctuations) with Ti4+ doping (antiferromagnetic fluctuations) in tuning spin-fluctuation spectra.
• To establish that chemical substitution can band-selectively modify spin fluctuations in a multi-band correlated metal like Sr2RuO4.

Proposed method

• Performed high-precision measurements of specific heat, electrical resistivity, and static magnetic susceptibility in Sr2-yLayRuO4 for y = 0 to 0.27.
• Used a realistic tight-binding model with rigid-band shift to map the Fermi energy evolution toward the van Hove singularity (vHS) of the γ-band.
• Calculated the Lindhard susceptibility to analyze the nesting vector evolution of the γ-band Fermi surface with doping.
• Compared the observed non-Fermi-liquid behavior (resistivity exponent n ≈ 1.4) with theoretical predictions for two-dimensional ferromagnetic spin fluctuations.
• Contrasted results with Ti4+ substituted Sr2Ru1-xTixO4 to highlight band-selective effects on spin fluctuations.
• Analyzed the evolution of the Fermi surface topology and density of states (DOS) using experimental Fermi surface data from de Haas-van Alphen and ARPES.

Experimental results

Research questions

• RQ1 How does La3+ substitution for Sr2+ in Sr2RuO4 modify the electronic structure and spin-fluctuation spectrum?
• RQ2 What is the role of the γ-band Fermi surface in driving non-Fermi-liquid behavior near a van Hove singularity?
• RQ3 Why does La-doping induce ferromagnetic spin fluctuations while Ti-doping induces antiferromagnetic order in Sr2RuO4?
• RQ4 How does the nesting vector of the γ-band evolve with electron doping, and what is its impact on magnetic susceptibility?
• RQ5 Can chemical doping selectively enhance spin fluctuations in specific Fermi surface sheets in a multi-band correlated metal?

Key findings

• The electronic specific heat coefficient increases gradually with La doping, indicating enhanced density of states (DOS) near the Fermi level.
• The static magnetic susceptibility increases more drastically with doping, signaling enhanced spin fluctuations, particularly near y ≈ 0.20.
• A breakdown of quasi-two-dimensional Fermi-liquid behavior is observed near y_c ≈ 0.20, marked by a resistivity exponent n ≈ 1.4.
• The enhancement of DOS and susceptibility is attributed to the Fermi energy approaching a van Hove singularity (vHS) in the γ-band, located 49 meV below the vHS in pristine Sr2RuO4.
• The nesting vector of the γ-band shrinks from Q_ic^γ ≈ (0.2π, 0.2π, 0) at y = 0 to Q_ic^γ ≈ 0 at y ≈ 0.20, promoting ferromagnetic spin fluctuations.
• Non-Fermi-liquid behavior at y ≈ 0.20 is consistent with two-dimensional ferromagnetic spin fluctuations, as predicted by the self-consistent renormalization theory with n = 4/3, and matches the observed n ≈ 1.4.