We report the temperature T and magnetic field H dependence of the thermopower S of an itinerant triangular antiferromagnet PdCrO 2 in high magnetic fields up to 32 T. In the paramagnetic phase, the zerofield thermopower is positive with a value typical of good metals with a high carrier density. In marked contrast to typical metals, however, S decreases rapidly with increasing magnetic field, approaching zero at the maximum field scale for T > 70 K. We argue here that this profound change in the thermoelectric response derives from the strong interaction of the 4d correlated electrons of the Pd ions with the shortrange spin correlations of the Cr 3þ spins that persist beyond the Néel ordering temperature due to the combined effects of geometrical frustration and low dimensionality. DOI: 10.1103/PhysRevLett.116.087202 The interplay between itinerant electrons and even simple magnetic structures can lead to spectacular effects, the giant magnetoresistance seen in magnetic multilayers being arguably the most prominent example [1]. In geometrically frustrated magnets, complex spin textures that couple to the conduction electrons create an altogether different landscape, where short-range correlations are expected to play a major role. Moreover, since magnetism in metals can be destabilized much more readily than in insulators, magnetic frustration in metallic systems offers a rich playground to search for the emergence of novel transport phenomena. Notable recent examples include the unconventional anomalous Hall effect (AHE) observed in magnetic pyrochlores [2,3] and the suppression of thermopower in a longitudinal magnetic field in the layered Curie-Weiss metal Na x CoO 2 [4].Despite their obvious potential for new physics, metallic frustrated magnets have been noticeably less studied than their insulating counterparts, largely due to the fact that such materials are rare. Of particular interest are materials in which the conduction electrons and magnetic moments arise from different subsystems. In this context, the quasi-two-dimensional (quasi-2D) antiferromagnet PdCrO 2 [4-6] is somewhat unique. PdCrO 2 has a delafossite crystal structure with layers of Pd ions arranged in a triangular lattice stacked between magnetic edgesharing CrO 6 octahedra. The latter contains Cr 3þ ions with localized (Mott insulating) 3=2 spins which order in the 120°antiferromagnetic (AFM) structure below T N ¼ 37.5 K [7,8]. The frustration parameter f, defined as an absolute ratio of the Weiss temperature Θ W and the ordering temperature T N , is around 13 for PdCrO 2 , indicating a high level of frustration [7][8][9]. According to band structure calculations [10], angle-resolved photoemission [10], and quantum oscillation (QO) studies [11,12],
In many layered metals, coherent propagation of electronic excitations is often confined to the highly conducting planes. While strong electron correlations and/or proximity to an ordered phase are believed to be the drivers of this electron confinement, it is still not known what triggers the loss of interlayer coherence in a number of layered systems with strong magnetic fluctuations, such as cuprates. Here, we show that a definitive signature of interlayer coherence in the metallic-layered triangular antiferromagnet PdCrO2 vanishes at the Néel transition temperature. Comparison with the relevant energy scales and with the isostructural non-magnetic PdCoO2 reveals that the interlayer incoherence is driven by the growth of short-range magnetic fluctuations. This establishes a connection between long-range order and interlayer coherence in PdCrO2 and suggests that in many other low-dimensional conductors, incoherent interlayer transport also arises from the strong interaction between the (tunnelling) electrons and fluctuations of some underlying order.
Using the Ong construction for a two-dimensional metal, we show that the sign change in the Hall coefficient R H of underdoped hole-doped cuprates at low temperature is consistent with the emergence of biaxial charge order recently proposed to explain the observation of low-frequency quantum oscillations. The sharp evolution of R H with temperature, however, can only be reconciled by incorporating a highly anisotropic quasiparticle scattering rate. The magnitude and form of the scattering rate extracted from the fitting imply that those quasiparticles at the vertices of the reconstructed pocket(s) approach the boundary of incoherence at the onset of charge order.
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