Ca3Ru2O7: Electronic instability and extremely strong quasiparticle renormalisation

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Abstract

We report on the electronic structure of the bilayer ruthenate Ca3Ru2O7 using high quality single-crystals grown by a floating-zone method which have the lowest residual resistivity so far achieved. The quantum oscillation, specific heat, and angle-resolved photoemission-spectroscopy (ARPES) measurements performed on these crystals establish that Ca3Ru2O7 is a quasi two-dimensional low-carrier metal with a density-wave instability.

Introduction

The layered perovskite ruthenates have generated a wealth of novel physics since the discovery of superconductivity in Sr2RuO4 [1]. The superconducting order parameter has been shown to be unconventional [2], and extensive investigations of related compounds such as Ca2RuO4, SrRuO3, Sr3Ru2O7 and Sr4Ru3O10 have revealed a number of novel phase transitions and magnetic states [3], [4], [5], [6], [7]. Among the series, the properties of the bilayer material Ca3Ru2O7 have presented something of a mystery. Single crystals have been grown by both the flux method [8] and in image furnaces [9]. The measured resistivity (ρ) shows metallic (dρ/dT>0) behaviour at low temperatures with a residual resistivity of 400μΩcm in Ref. [9], but flux-grown crystals present a non-metallic temperature dependence (dρ/dT<0) [8]. Besides, the electronic specific heat (γ) has been reported to have a metallic component that varies considerably, from 1.7mJ/K2mol-Ru [9] to 20mJ/K2 mol-Ru [10]. Shubnikov–de Haas oscillations have been seen by both groups, but the only direct comparisons of the specific heat coefficient with the quasiparticle masses gave very poor agreement [11], leaving many open questions about the precise nature of the ground state of the material. In this paper, we describe the growth, in an image furnace, of metallic single crystals of Ca3Ru2O7 with a residual resistivity one order of magnitude lowerthan the best previously observed value. Using these crystals, we have performed detailed electronic structure studies by means of quantum oscillations and high-resolution angle-resolved photoemission-spectroscopy (ARPES).

Section snippets

Experiment

The Ca3Ru2O7 crystals were grown by a floating-zone method with RuO2-rich feed rods. The optimised feed rods for our image furnace were prepared with a composition of Ca2RunOy(n=2) reflecting a heavy evaporation of RuO2 during the crystal growth. The growth was done at the speed of 7 mm/h and under a pressure of 1 MPa with a flowing gas mixture (Ar:O2=9:1). Crystals with the typical size of 3mm×4mm×1mm(caxis) were obtained. The crystals were characterised by X-ray and SEM (scanning electron

Transport properties and electronic structure

The in-plane resistivity ρab of two of the resulting crystals is plotted in Fig. 1 as a function of temperature down to 4 K. The most prominent features are associated with the previously-established Néel and first order structural phase transitions denoted as TN and TS [8], [9], [12], [13]. At temperatures above these transitions, ρab is almost linear in T.

Approximately below 30 K, ρab drops dramatically, giving evidence for a strongly metallic temperature dependence. Using these metallic

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1

Present address: National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan.

2

Present address: School of Physics and Astronomy, University of St. Andrews, Fife KY16 9SS, UK.

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