No abstract

Studies of low-frequency resistance noise show that the glassy freezing of the two-dimensional (2D) electron system in the vicinity of the metal-insulator transition occurs in all Si inversion layers. The size of the metallic glass phase, which separates the 2D metal and the (glassy) insulator, depends strongly on disorder, becoming extremely small in high-mobility samples. The behavior of the second spectrum, an important fourth-order noise statistic, indicates the presence of long-range correlations between fluctuators in the glassy phase, consistent with the hierarchical picture of glassy dynamics.

No abstract

In the underdoped pseudogap regime of cuprate superconductors, the normal state is commonly probed by applying a magnetic field (H). However, the nature of the H-induced resistive state has been the subject of a long-term debate, and clear evidence for a zero-temperature (T = 0) H-tuned superconductor-insulator transition (SIT) has proved elusive. Here we report magnetoresistance measurements in underdoped La 2−x Sr x CuO 4 , providing striking evidence for quantum critical behavior of the resistivity -the signature of a H-driven SIT. The transition is not direct: it is accompanied by the emergence of an intermediate state, which is a superconductor only at T = 0. Our finding of a two-stage H-driven SIT goes beyond the conventional scenario in which a single quantum critical point separates the superconductor and the insulator in the presence of a perpendicular H. Similar two-stage H-driven SIT, in which both disorder and quantum phase fluctuations play an important role, may also be expected in other copper-oxide high-temperature superconductors.The SIT is an example of a quantum phase transition (QPT): a continuous phase transition that occurs at T = 0, controlled by some parameter of the Hamiltonian of the system, such as doping or the external magnetic field 1 . A QPT can affect the behavior of the system up to surprisingly high temperatures. In fact, many unusual properties of various strongly correlated materials have been attributed to the proximity of quantum critical points (QCPs). An experimental signature of a QPT at nonzero T is the observation of scaling behavior with relevant parameters in describing the data. Although the SIT has been studied extensively 2 , even in conventional superconductors many questions remain about the perpendicular-field-driven SIT in two-dimensional (2D) or quasi-2D systems 3 . In high-T c cuprates (T c -transition temperature), which have a quasi-2D nature, early magnetoresistance (MR) experiments showed the suppression of superconductivity with high H, revealing the insulating behavior 4-6 and hinting at an underlying H-field-tuned SIT 7 . However,

We report on the observation of the Ising quantum Hall ferromagnet with Curie temperature T(C) as high as 2 K in a modulation-doped (Cd,Mn)Te heterostructure. In this system field-induced crossing of Landau levels occurs due to the giant spin-splitting effect. Magnetoresistance data, collected over a wide range of temperatures, magnetic fields, tilt angles, and electron densities, are discussed taking into account both Coulomb electron-electron interactions and s-d coupling to Mn spin fluctuations. The critical behavior of the resistance "spikes" at T-->T(C) corroborates theoretical suggestions that the ferromagnet is destroyed by domain excitations.

The relaxations of conductivity have been studied in a strongly disordered two-dimensional (2D) electron system in Si after excitation far from equilibrium by a rapid change of carrier density ns at low temperatures T. The dramatic and precise dependence of the relaxations on ns and T strongly suggests (a) the transition to a glassy phase as T-->0, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics.

Studies of low-frequency resistance noise show that the dramatic change in the dynamics of the twodimensional electron system (2DES) in Si that occurs near the metal-insulator transition (MIT) persists in high parallel magnetic fields B such that the 2DES is fully spin polarized. This strongly suggests that charge, as opposed to spin, degrees of freedom are responsible for this effect. In the metallic phase, however, noise is suppressed by a parallel B, pointing to the role of spins. At low B, the temperature dependence of conductivity in the metallic phase provides evidence for a MIT. DOI: 10.1103/PhysRevLett.92.226403 PACS numbers: 71.30.+h, 64.70.Pf, 71.27.+a, 73.40.Qv The fascinating strong correlation physics exhibited by low-density two-dimensional (2D) electron and hole systems [1] has been the subject of intensive research. In particular, the precise role of the spin degrees of freedom remains the central unresolved issue in this field [2]. Recent resistance (R) noise measurements on a 2D electron system in Si [3,4] have revealed a striking change in the dynamics of the two-dimensional electron system (2DES) in the vicinity of the apparent metal-insulator transition (MIT), posing a question of whether charge or spin degrees of freedom are responsible for this effect. Since it is known [5] that a relatively weak magnetic field B is required to fully spin polarize the 2DES in the relevant range of densities n s , experimental studies in parallel B [6] should be able to distinguish between the two possibilities. Here we present such a study, which shows that the dramatic change in the electron dynamics persists even when the 2DES is spin polarized, strongly pointing to charge, as opposed to spin, degrees of freedom as the origin of this effect. At high n s , on the other hand, noise measurements in B suggest that electrons' spins may play a relevant role.The B 0 studies employed a combination of transport and low-frequency resistance noise measurements [3,4] to probe the system dynamics. By reducing n s , it was found that, at some well-defined density n g , (i) the dynamics suddenly and dramatically slowed down, and (ii) there was an abrupt change to the sort of correlated statistics characteristic of complicated multistate systems. These features were attributed to the glassy freezing of the 2DES at n g , with the data being consistent with the hierarchical picture of glassy dynamics. Hence, for brevity, this change in the noise behavior will be called the ''glass transition.'' The ''glass transition'' occurs [3] in the metallic phase, i.e., at n g > n c , where n c is the critical density for the MIT determined from the vanishing of activation energy in the insulating regime [7,8]. The intermediate metallic phase with slow electron dynamics (MSED, or ''metallic glass'') is considerably wide in strongly disordered samples [n g ÿ n c =n c 0:5 [3]], whereas in devices with low disorder n g is at most a few percent higher than n c [4]. In this work, we investigate these same high peak mobility (), i.e., ...

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...