www.nature.com/npjquantmats ARTICLE OPEN Metallicity without quasi-particles in room-temperature strontium titanate Xiao Lin1, Carl Willem Rischau1, Lisa Buchauer1, Alexandre Jaoui1, Benoît Fauqué1,2 and Kamran Behnia1 Cooling oxygen-deficient strontium titanate to liquid-helium temperature leads to a decrease in its electrical resistivity by several orders of magnitude. The temperature dependence of resistivity follows a rough T3 behavior before becoming T2 in the low- temperature limit, as expected in a Fermi liquid. Here, we show that the roughly cubic resistivity above 100 K corresponds to a regime where the quasi-particle mean-free-path is shorter than the electron wave-length and the interatomic distance. These criteria define the Mott-Ioffe-Regel limit. Exceeding this limit is the hallmark of strange metallicity, which occurs in strontium titanate well below room temperature, in contrast to other perovskytes. We argue that the T3-resistivity cannot be accounted for by electron-phonon scattering à la Bloch–Gruneisen and consider an alternative scheme based on Landauer transmission between individual dopants hosting large polarons. We find a scaling relationship between carrier mobility, the electric permittivity and the frequency of transverse optical soft mode in this temperature range. Providing an account of this observation emerges as a challenge to theory. npj Quantum Materials (2017)2:41 ; doi:10.1038/s41535-017-0044-5 INTRODUCTION interatomic distance, the lowest conceivable length scale. The existence of well-defined quasi-particles is taken for granted Strontium titanate is the strangest known metal according to in the Boltzmann–Drude picture of electronic transport. In this the criteria for strangeness widely used (See Table 1). Since quasi- picture, carriers of charge or energy are scattered after traveling a particles in this system are not well-defined in a time scale long finite distance. The Mott-Ioffe-Regel (MIR) limit is attained when enough to allow the existence of a scattering time, charge the mean-free-path of a carrier falls below its Fermi wavelength or transport cannot be reasonably described by a phonon-scattering 16–19, 23–25 the interatomic distance.1 In most metals, resistivity saturates picture. This was the approach in previous attempts to when this limit is approached. But in “bad”2 or “strange”3 metals, it understand charge transport. We argue that metallicity is caused continues to increase.4 It is indeed strange when the mean-free- by the temperature dependence of the transmission coefficient path persists to fall after attaining its shortest conceivable along Landauer canals between dopants. Such a picture does not magnitude and often unexpected behavior is considered bad. require a scattering time or a mean-free-path. We find an In most cases, bad metals present a linear temperature empirical link between mobility and permittivity, which gives a dependence beyond the MIR limit. Bruin et al.5 recently noticed reasonable account of the experimental data. Providing a that the T-linear scattering rate in numerous metals, either quantitative account of this observation emerges as a specific ordinary or strange, has a similar magnitude. This observation challenge to transport theory. h suggests the relevance of a universal Planck timescale (τP , kBT where kB is the Boltzmann and ħ the reduced Planck constants) to electronic dissipation and motivated a theoretical attempt to RESULTS quantify an upper limit to the diffusion constant in incoherent Metallicity beyond the MIR limit in strontium titanate metals.6 During the past years, several theories of charge transport Figure 1a shows the temperature dependence of resistivity in fi 7–14 17 with no need for well-de ned quasi-particles were proposed. SrTiO3−δ as the carrier concentration is changed from 2.4 × 10 More recently, direct measurements of thermal diffusivity15 have cm−3 to 3.5 × 1019 cm−3. At low temperature (T 100 K), the provided additional evidence for quasi-particle-free thermal system displays a T-square resistivity21, 22 as expected for electron- transport at room temperature in cuprates. electron scattering26 in a Fermi liquid. Plotting resistivity as a 2 We show here that n-doped SrTiO3 is an unnoticed case of function of T (see Fig. 2 in ref. 22), one can see that inelastic strange or bad metallicity. The resistivity of this dilute metal resistivity becomes T-square in the low-temperature limit. A wide decreases by orders of magnitude upon cooling to cryogenic range of dense Fermi liquids display Kadowaki–Woods scaling27 of – temperatures.16 20 In the low-temperature limit, resistivity follows their T-square resistivity prefactor and their electronic specific a T-square behavior,21, 22 as expected in a Fermi liquid. Above 100 heat. Since the latter is proportional to carrier concentration, the 17, 22 28 K, however, the temperature dependence is close to cubic. In case of low-density systems such as doped SrTiO3 is different. this regime, the mean-free-path of carriers becomes shorter than However, the scaling between the resistivity prefactor and their wavelength. At room temperature, it falls well below the Fermi energy persists. Indeed, the magnitude of this prefactor 1Laboratoire Physique et Etude de Matériaux (CNRS-UPMC), ESPCI Paris, PSL Research University, 75005 Paris, France and 2JEIP, USR 3573 CNRS, Collège de France, PSL Research University, 75005 Paris, France Correspondence: Kamran Behnia ([email protected]) Received: 23 February 2017 Revised: 16 June 2017 Accepted: 22 June 2017 Published online: 21 July 2017 Published in partnership with Nanjing University Quasi-particle-free metallicity of SrTiO3 X Lin et al. 2 Table 1. How the MIR limit of metallicity is exceeded in different perovskytes −1 ρ Ω ‘ ‘ Compound kF (nm ) (400 K) (m cm) (400 K) (nm) kF (400 K) TkF ‘¼1 (K) References La1.72Sr0.18Cu04 6.6 0.32 0.35 2.2 850 4, 63 YBa2Cu4O8 1.4 0.36 0.8 1.1 450 64, 65 Sr2RuO4 5.6 0.2 0.38 2.1 800 66 17 −3 SrTiO3−δ (n = 410 cm ) 0.24 5900 0.2 0.05 100 This work The table compares the Fermi wave-vector, the resistivity, and the mean-free-path at 400 K in four different systems using Eq. (1). The table lists an underdoped, an optimally-doped cuprate and strontium ruthenate and compares them with doped strontium titanate. In the case of YBa2Cu4O8, we assume that there are four Fermi surface pockets with a kF corresponding to the square root of the frequency of quantum oscillations. In contrast to other systems, the MIR limit in doped strontium titanate occurs well below room temperature. Fig. 2 Breakdown of the Mott-Ioffe-Regel limit. a Temperature Fig. 1 Resistivity and mobility in doped SrTiO3. a Temperature dependence of the mean-free-path in dilute metallic SrTiO3−δ dependence of resistivity in SrTiO −δ as the carrier concentrations is − 3 sample up to 400 K. Blue circles represent ‘ , extracted from tuned from 1017 to 1019 cm 3. Room-temperature resistivity is e resistivity using Eq. (1). Red diamonds represent the mean-free-path several hundred times higher than low-temperature resistivity. b À = ‘′ ¼ ‘ n 1 3 in the non-degenerate regime: e e Λ . Also shown are the de Hall mobility as a function of temperature. Above 100 K, the −1/3 mobility does not depend on carrier concentration and is roughly Broglie thermal wavelength, Λ, the inter-electron distance n , the À1 = cubic in temperature. c Assuming that inelastic resistivity follows a inverse of the Fermi wavelength (kF ) and the lattice parameter, a α power law, i.e., ρ = ρ0 + AT , one can extract the exponent, α by 0.39 nm. The system remains metallic even at 400 K, in spite of taking a logarithmic derivative: α = dln(ρ − ρ0)/dlnT, as in the case of exceeding the MIR limit by all conceivable criteria. b A color 62 α ≃ ‘′ cuprates. At low temperature, 2. Above 50 K, it starts a plot of e, extracted from the resistivity data of Fig. 1 in the (T, n) significant shift upward and exceeds three around 150 K, before plane. Different crossovers are shown. The MIR limit is exceeded in a steadily decreasing afterwards. The antiferrodistortive transition39 is high-temperature window narrowing down with increasing the source of the small anomaly at 105 K concentration npj Quantum Materials (2017) 41 Published in partnership with Nanjing University Quasi-particle-free metallicity of SrTiO3 X Lin et al. 3 decreases by several orders of magnitude with changing carrier As seen in Fig. 2a, the mean-free-path becomes shorter than the 22 À1 concentration. inverse of the Fermi wave-vector kF and then the lattice Figure 1b shows the temperature dependence of the extracted parameter a. Now, at this carrier concentration, with an effective μ ¼ 1 * Hall mobility, neρ. Above 100 K, it displays a temperature mass of m = 1.8me, the Fermion degeneracy temperature is as dependence close to T−3 and barely varies with carrier concentra- low as 15 K.32 As one sees in the figure, this is the temperature at tion. Our data is in agreement with the less extensive data ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffih 17 which de Broglie thermal wavelength, π Ã becomes reported previously by Tufte and Chapman. Figure 1c shows 2 m kBT −1/3 how the exponent of inelastic resistivity evolves as a function of comparable to the inter-electron distance n . Above this α temperature, the electrons become non-degenerate and follow temperature. The total resistivity can be expressed as ρ = ρ0 + AT . As seen in Fig. 1c, α is 2 at very low-temperature (T <50K) but a Maxwell–Boltzmann distribution. As a consequence and thanks smoothly rises with warming. One does not expect the T-square to thermal excitation, the typical carrier momentum is larger than ħ resistivity to survive at the Fermi degeneracy temperature.