S. W. Cheong

Bio: S. W. Cheong is an academic researcher from Alcatel-Lucent. The author has contributed to research in topics: Chalcogenide glass & Magnetoresistance. The author has an hindex of 19, co-authored 35 publications receiving 4380 citations.

Spin-Polarized Intergrain Tunneling in La 2/3 Sr 1/3 MnO 3

Abstract: The magnetoresistance (MR) and the field dependent magnetization have been systematically examined in the low temperature ferromagnetic metallic state of single crystal and polycrystalline ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$. We find that the intrinsic negative MR in single crystal is due to the suppression of spin fluctuations, and magnetic domain boundaries do not dominate the scattering process. In contrast, we demonstrate that the MR in the polycrystalline samples exhibits two distinct regions: large MR at low fields dominated by spin-polarized tunneling between grains and high field MR which is remarkably temperature independent from 5 to 280 K.

Spin waves and electronic interactions in La2CuO4

Abstract: The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.

Commensurate to incommensurate charge ordering and its real space images in La0.5Ca0.5MnO3.

Abstract: The antiferromagnetic to ferromagnetic transition in L${\mathrm{a}}_{0.5}$C${\mathrm{a}}_{0.5}$Mn${\mathrm{O}}_{3}$ is accompanied by a commensurate to incommensurate charge-ordering transition. The presence of incommensurate charge correlation in the ferromagnetic phase is a remarkable interplay between charge and spin in double-exchange ferromagnets. Furthermore, a dark-field imaging technique has provided real-space images of charge-ordered domains revealing discommensurations inside the nearly commensurate domains. Local density fluctuations of discommensurations are responsible for the similar variations of the charge-ordering wave vector.

Incommensurate magnetic fluctuations in La2-xSrxCuO4.

Abstract: We use inelastic neutron scattering to establish the modulation vectors \ensuremath{\delta} and correlation lengths for the incommensurate magnetic fluctuations in metallic samples of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ with x=0.075 and 0.14. In notation appropriate for a square lattice where the magnetic instability in the undoped case occurs at (\ensuremath{\pi},\ensuremath{\pi}), the vectors \ensuremath{\delta} are along (\ensuremath{\pi},0) and (0,\ensuremath{\pi}). The correlation length \ensuremath{\xi} is larger than the distance between carriers, is weakly dependent on x, and changes significantly between 12 and 100 K for both compositions.

Thermodynamic and electron diffraction signatures of charge and spin ordering in La1-xCaxMnO3

Abstract: The large-magnetoresistance compounds ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ have been studied using specific heat, sound velocity $(v)$, and electron diffraction. For $0.63\ensuremath{\le}x\ensuremath{\le}0.67$ charge ordering is observed at 260 K and accompanied by a dramatic $(g10%)$ increase in $v$. This simultaneous occurrence of electron and lattice ordering features implies extremely strong electron-phonon coupling, known to exist for the octahederally coordinated ${d}^{4}$ ion and originating in the Jahn-Teller effect. A dynamic manifestation of this Jahn-Teller coupling has been suggested by Millis et al. as the origin for colossal magnetoresistance.

Doping a Mott insulator: Physics of high-temperature superconductivity

Abstract: This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

Why Are There so Few Magnetic Ferroelectrics

Abstract: Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Evidence for stripe correlations of spins and holes in copper oxide superconductors

Abstract: ONE of the long-standing mysteries associated with the high-temperature copper oxide superconductors concerns the anomalous suppression1 of superconductivity in La2-xBaxCuO4 (and certain related compounds) when the hole concentration x is near . Here we examine the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-xSrxCuO4 by neutron scattering2–4. A possible explanation for the incommensurability involves a coupled, dynamical modulation of spin and charge in which antiferromagnetic 'stripes' of copper spins are separated by periodically spaced domain walls to which the holes segregate5–9. An ordered stripe phase of this type has recently been observed in hole-doped La2NiO4 (refs 10–12). We present evidence from neutron diffraction that in the copper oxide material La1.6-xNd0.4SrxCuO4, with x = 0.12, a static analogue of the dynamical stripe phase is present, and is associated with an anomalous suppression of superconductivity13,14. Our results thus provide an explanation of the ' ' conundrum, and also support the suggestion15 that spatial modulations of spin and charge density are related to superconductivity in the copper oxides.

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

Abstract: Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.