arXiv subjects

Ching-Hao Chang

Publications and source records attributed to Ching-Hao Chang.

28 records · Page 2Linked to original sources

arXiv2017

Carrier driven coupling in ferromagnetic oxide heterostructures

Transition metal oxides are well known for their complex magnetic and electrical properties. When brought together in heterostructure geometries, they show particular promise for spintronics and colossal magnetoresistance applications. In this letter, we propose a new mechanism for the coupling between layers of itinerant ferromagnetic materials in heterostructures. The coupling is mediated by charge carriers that strive to maximally delocalize through the heterostructure to gain kinetic energy. In doing so, they force a ferromagnetic or antiferromagnetic coupling between the constituent layers. To illustrate this, we focus on heterostructures composed of SrRuO$_3$ and La$_{1-x}$A$_{x}$MnO$_3$ (A=Ca/Sr). Our mechanism is consistent with antiferromagnetic alignment that is known to occur in multilayers of SrRuO$_3$-La$_{1-x}$A$_{x}$MnO$_3$. To support our assertion, we present a minimal Kondo-lattice model which reproduces the known magnetization properties of such multilayers. In addition, we discuss a quantum well model for heterostructures and argue that the spin-dependent density of states determines the nature of the coupling. As a smoking gun signature, we propose that bilayers with the same constituents will oscillate between ferromagnetic and antiferromagnetic coupling upon tuning the relative thicknesses of the layers.

cond-mat.str-el↗

arXiv2016

Power-law ansatz in complex systems: excessive loss of information

The ubiquity of power-law relations in empirical data displays physicists' love of simple laws and uncovering common causes among seemingly unrelated phenomena. However, many reported power laws lack statistical support and mechanistic backings, not to mention discrepancies with real data are often explained away as corrections due to finite size or other variables. We propose a simple experiment and rigorous statistical procedures to look into these issues. Making use of the fact that the occurrence rate and pulse intensity of crumple sound obey power law with an exponent that varies with material, we simulate a complex system with two driving mechanisms by crumpling two different sheets together. The probability function of crumple sound is found to transit from two power-law terms to a {\it bona fide} power law as compaction increases. In addition to showing the vicinity of these two distributions in the phase space, this observation nicely demonstrates the effect of interactions to bring about a subtle change in macroscopic behavior and more information may be retrieved if the data are subject to sorting. Our analyses are based on the Akaike information criterion that is a direct measurement of information loss and emphasizes the need to strike a balance between model simplicity and goodness of fit. As a show of force, the Akaike information criterion also found the Gutenberg-Richter law for earthquakes and the scale-free model for brain functional network, 2-dimensional sand pile, and solar flare intensity to suffer excessive loss of information. They resemble more the crumpled-together ball at low compactions in that there appear to be two driving mechanisms that take turns occurring.

physics.soc-ph↗

arXiv2015

Engineering the interlayer exchange coupling in magnetic trilayers

When the thickness of metal film approaches the nanoscale, itinerant carriers resonate between its boundaries and form quantum well states (QWSs), which are crucial to account for the film electrical, transport and magnetic properties. Besides the classic origin of particle-in-a-box, the QWSs are also susceptible to the crystal structures that affect the quantum resonance. Here we investigate the QWSs and the magnetic interlayer exchange coupling (IEC) in the Fe/Ag/Fe (001) trilayer from first-principles calculations. We find that the carriers at the Brillouin-zone center (belly) and edge (neck) separately form electron- and hole-like QWSs that give rise to an oscillatory feature for the IEC as a function of the Ag-layer thickness with long and short periods. Since the QWS formation sensitively depends on boundary conditions, one can switch between these two IEC periods by changing the Fe-layer thickness. These features, which also occur in the magnetic trilayers with other noble-metal spacers, open a new degree of freedom to engineer the IEC in magnetoresistance devices.

cond-mat.mtrl-sci↗

arXiv2014

Strongly anisotropic ballistic magnetoresistance in compact three-dimensional semiconducting nanoarchitectures

We establish theoretically that in nonmagnetic semiconducting bilayer or multilayer thin film systems rolled up into compact quasi-one-dimensional nanoarchitectures, the ballistic magnetoresistance is very anisotropic: conductances depend strongly on the direction of an externally applied magnetic field. This phenomenon originates from the curved open geometry of rolled-up nanotubes, which leads to a tunability of the number of quasi-one-dimensional magnetic subbands crossing the Fermi energy. The experimental significance of this phenomenon is illustrated by a sizable anisotropy that scales with the inverse of the winding number, and persists up to a critical temperature that can be strongly enhanced by increasing the strength of the external magnetic field or the characteristic radius of curvature, and can reach room temperature.

cond-mat.mes-hall↗

arXiv2012

Switching off the magnetic exchange coupling by quantum resonances

We clarify the role of quantum-well states in magnetic trilayer systems from majority carrier in the ferromagnetic and all carriers in the antiferromagnetic configurations. In addition to numerical and analytic calculations, heuristic pictures are provided to explain effects of a capping layer and side-layer modulation in recent experiments. This immediately offers answers to two unexplained subtle findings in experiments and band-structure calculations, individually. Furthermore, it allows a more flexible tuning of or even turning off the interlayer exchange coupling.

cond-mat.mtrl-sci↗

arXiv2012

Analogy of RKKY oscillations to the heat exchange in cold atoms

An oscillatory term is found in both the energy expectation and dynamics of a wave-packet in a time-varying harmonic trap and infinite potential well. They are proved to oscillate in coherence with the time lapse within each period depending on both the cutoff in transition energies and the specific route via which the potential is being varied. This oscillatory term is general to arbitrary potential forms since it derives from the interference between crossed transition trajectories. Close analogy is made to the Ruderman-Kittel-Kasuya-Yosida interaction for giant-magnetoresistance trilayers, where many-body quantum interference among scattering states renders the oscillation as a function of spacer width. This connection reveals the generality of quantum friction due to parasitic oscillations.

cond-mat.quant-gas↗

arXiv2010

Interlayer Exchange Coupling Beyond the Proximity Force Approximation

Ion bombardment has been shown to be capable of enhancing the interlayer exchange coupling in a trilayer system that exhibits giant magnetoresistance. We demonstrate that this phenomenon can be derived from the phase coherence among scattered paths within the two rough interfaces when their topographies are correlated. In the case of mild corrugations, our method reproduces the predictions by the proximity force approximation which does not consider the interference. When the characteristic Fourier conjugate of the tomography becomes large and comparable to the Fermi momentum, interesting new features arise and can only be captured by our more general approach. Among our findings, the scenario of an enhanced interlayer exchange coupling due to the interface roughness is explained, along with how it depends on the sample parameters. An additional channel for the resonant transmission is identified due to extra scattering paths from the roughness.

cond-mat.mes-hall↗

arXiv2009

Origin of branch points in the spectrum of PT-symmetric periodic potentials

There exists multiple branch points in the energy spectrum for some \emph{PT}-symmetric periodic potentials, where the real eigenvalues turn into complex ones. By studying the transmission amplitude for a localized complex potential, we elucidate the physical origin of the breakdown of perturbation method and Born approximation. Mostly importantly, we derive an analytic criteria to determine why, when and where the bifurcation will occur.

quant-ph↗

arXiv2009

Spin-glass-like behavior caused by Mn-rich Mn(Ga)As nanoclusters in GaAs

We simulate the indirect exchange interaction between Mn-rich Mn(Ga)As nanoclusters in GaAs by analytical means. In contrast to the conventional Ruderman-Kittel-Kasuya-Yosida (RKKY) formula which considers the mediation by the carriers in the medium, we also include the contribution from those inside the clusters. Since the carrier concentration is higher in the clusters, this modification allows the RKKY oscillation to change sign. Consequently, while the previous approach only favors ferromagnetism for this system, an antiferromagnetic coupling is in fact possible. Since the Mn-rich Mn(Ga)As nanoclusters are naturally formed and bound to have different sizes, their spin orientation is likely to be frustrated due to mixed preferences from different neighbors. We argue that this is likely the source of the spin-glass-like behavior which plagues this system. By tuning the size and narrowing its distribution, normal ferromagnetism can be restored with an Curie temperature higher than previously thought.

cond-mat.mtrl-sci↗

arXiv2009

Interlayer coupling enhanced by the interface roughness

Previous experiment on Fe/Cr/Fe(001) trilayers reported a surprising observation that the interlayer exchange coupling could be enhanced drastically by the bombardment of irradiation even at low fluences. We propose that it is due to the resonant states in the spacer, made possible when the topography of both interfaces is correlated and exhibits prominent Fourier components. A systematic procedure is developed to handle the interface roughness and predict on how to optimize the interlayer coupling. This method can be extended to bridge the gap between theories and experiments in other heterojunctions.

cond-mat.dis-nn↗