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A c-axis magnetotransport and resistance noise study in La1.97Sr0.03CuO4 reveals clear signatures of glassiness, such as hysteresis, memory, and slow, correlated dynamics, but only at temperatures (T ) well below the spin glass transition temperature Tsg. The results strongly suggest the emergence of charge glassiness, or dynamic charge ordering, as a result of Coulomb interactions. PACS numbers: 74.72.Dn, 72.70.+m, 75.50.Lk The role of heterogeneities observed in most holedoped high-temperature superconductors (HTS) is one of the major open issues in the field [1,2]. In weakly doped Mott insulators, such as HTS, charge heterogeneities are expected to arise due to the existence of several competing ground states [3], and may be even to exhibit glassy dynamics [4]. Even a small amount of disorder may favor glassiness over various static charge-ordered states [5]. Experiments in hole-doped HTS suggest [6,7,8,9] that glassiness of both spins and charges emerges with the first added holes and evolves with doping x. While spin glass (SG) behavior is well established at low T , the evidence for glassy freezing of charges is not conclusive. Hence, alternative, bulk probes of charge dynamics are needed to explore the nature of the ground state. We present a novel study of the charge dynamics in a lightly doped La 2−x Sr x CuO 4 (LSCO) using a combination of transport and noise spectroscopy that proved to be a powerful probe of dynamics in other glasses [10,11]. We find several clear signatures of glassiness at T ≪ T sg . The data strongly suggest that the doped holes form a dynamically ordered, cluster glass as a result of Coulomb interactions.In LSCO, the prototypical cuprate HTS, the threedimensional (3D) long range antiferromagnetic (AF) order of the parent compound is destroyed above x ≈ 0.02, but 2D short range AF correlations persist [12]. In particular, as a result of hole doping, CuO 2 (ab) planes develop a pattern of AF domains that are separated by antiphase boundaries [13,14,15]. Since the Dzyaloshinskii-Moriya interaction induces slight canting of the spins in CuO 2 planes towards the c axis, there is a weak ferromagnetic (FM) moment in the bc plane associated with each AF domain, such that the direction of the FM moment is uniquely linked to the phase of the AF order [15,16]. The interplane exchange favors staggered ordering of those FM moments in the c direction. At low enough T < T sg (x), the system freezes into a SG [12,14,15] that extends into the superconducting (SC) phase for x > 0.05[17] up to optimal doping [18] [ Fig. 1(a)]. Various experiments on lightly doped LSCO (e.g. Refs. [6,7]), including transport studies [19], were interpreted in terms of the hole-poor AF domains separated by the hole-rich regions in CuO 2 planes, with infrared studies being inconsistent with the notion of static charge ordering [8].We report an extensive study of the low-T (T ∼ 1 K and below) c-axis magnetotransport and low-frequency resistance (R) noise in LSCO with x = 0.03 [ Fig. 1(a)]. Such lightly doped samples a...

Magnetization and magnetoresistance have been measured in insulating antiferromagnetic La2Cu0.97Li0.03O4 over a wide range of temperatures, magnetic fields, and field orientations. The magnetoresistance step associated with a weak ferromagnetic transition exhibits a striking nonmonotonic temperature dependence, consistent with the presence of skyrmions.PACS numbers: 75.47. Lx,72.20.My,75.50.Ee A remarkable manifestation of complexity in magnetic systems is the emergence of topologically nontrivial arrangements of spins, such as skyrmions [1]. These are "knots" in an otherwise ordered spin texture, which behave as excitations with particlelike properties. Skyrmions are stabilized by a magnetic field B in several ferromagnetic (FM) metals [2][3][4][5], where they manifest themselves in the electronic transport [2-4, 6, 7], or they form a detectable periodic skyrmion lattice [5]. Skyrmions have been predicted to emerge also in the ground state of doped antiferromagnetic (AFM) insulators [8][9][10], but the identification of such isolated skyrmions is a challenge. Neutron scattering, for example, would not be a definitive probe, since skyrmions here do not form a lattice, while their possible signatures on transport may be screened by the insulating character of the carriers.In a FM system, a charge carrier with the spin aligned to the magnetic background preserves its metallic character with a mass renormalization due to scattering by lowenergy spin waves. Thus, the more complex spin excitations associated with topologically nontrivial magnetic textures have characteristic signatures in transport. This is indeed the case for skyrmions in the polarized quantum Hall state of a two-dimensional electron gas [2, 3], in colossal magnetoresistance (MR) manganites [4], and in the three-dimensional FM MnSi [6, 7]. In a doped AFM insulator, the description of transport is complex already in the topologically trivial AFM ground state, since the carriers cannot move without inducing spin-flip scattering [8, 11]. If, however, the external B field causes a change in transport that depends on the configuration of the spin background, the MR becomes a key probe to identify signatures of anomalous spin textures. This situation can be realized in the AFM insulator La 2 CuO 4 , lightly hole doped with Li. Here we show that the striking nonmonotonic temperature (T ) dependence of the MR step associated with a B-induced magnetic transition is