arXiv subjects

Ching-Hao Chang

Publications and source records attributed to Ching-Hao Chang.

At least 19 recordsLinked to original sources

arXiv2026

Electrically Controllable Flat Band in Two-Dimensional Electron Gases under Nonuniform Magnetic Fields

Flat bands underlie a diverse range of quantum phenomena, from strongly correlated phases to superconductivity. We theoretically establish that a two-dimensional electron gas under a linear magnetic-field gradient and a transverse electric field exhibits electrically tunable flat bands. When the electric field magnitude is tuned to a value within a discrete sequence, these bands become strictly dispersionless. By providing exact classical and quantum solutions, we demonstrate that these states are high-order Landau levels associated with drift-compensated cyclotron orbits of carriers arising from the synergy between the magnetic-field gradient and the electric field. These electrically controllable Landau levels exhibit quantized Hall conductance and a strongly enhanced density of states. Our results provide a new route for flat-band creation, magnetoelectric band engineering, and quantized Hall currents controlled via source-drain voltage.

cond-mat.mtrl-sci↗

arXiv2025

Bending nanoribbon to induce large anisotropic magnetoconductance

When a nanoribbon is bent under a homogeneous external magnetic field, the effective magnetic field inside becomes either homogeneous or inhomogeneous, depending on the direction of the field. This enables the selective creation of bulk, interface, and edge magnetic states in the bent structure, for a magnetic field with a strength. We establish theoretically that these tuneable states lead to a strong geometry-induced anisotropic magnetoconductance (GAMC) in perpendicularly bent nanoribbon, which can reach up to 100\%. Moreover, the GAMC can be further enhanced to 200\%, 300\%, or even higher by either further bending or tuning the bending angle. The potential of this phenomenon for practical applications is demonstrated by its stable anisotropy, which remains consistent across a wide range of Fermi energies, can be observed even at weak magnetic fields and room temperature, and occurs in various systems such as two-dimensional electron gas (2DEG) and graphene.

cond-mat.mes-hall↗

arXiv2025

Spectral bundles on Abelian varieties, complex projective spaces and Grassmannians

In this paper we study the spectral analysis of Bochner-Kodaira Laplacians on an Abelian variety, complex projective space $\mathbb{P}^{n}$ and a Grassmannian with a holomorphic line bundle. By imitating the method of creation and annihilation operators in physics, we convert those eigensections (of the \textquotedblleft higher energy" level) into holomorphic sections (of the \textquotedblleft lowest energy" level). This enables us to endow these spectral bundles, which are defined over the dual Abelian variety, with natural holomorphic structure. Using this conversion expressed in a concrete way, all the higher eigensections are explicitly expressible using holomorphic sections formed by theta functions. Moreover, we give an explicit formula for the dimension of the space of higher-level eigensections on $\mathbb{P}^{n}$ through vanishing theorems and the Hirzebruch-Riemann-Roch theorem. These give a theoretical study related to some problems newly discussed by string theorists using numerical analysis. Some partial results on Grassmannians are proved and some directions for future research are indicated.

math.DG↗

arXiv2025

Tuning the topological winding number by rolling up graphene

Nanoscrolls, radial superlattices formed by rolling up a nanomembrane, exhibit distinct electronic and magneto-transport properties compared to their flat counterparts. In this study, we theoretically demonstrate that the conductance can be precisely enhanced N times by rolling up graphene into an N-turn nanoscroll and applying a longitudinal magnetic field. This tunable positive magnetoconductance stems from the topological winding number which is activated in a carbon nanoscroll with magnetic flux and its maximum value purely increases with the scroll winding number (the number of turns). By integrating material geometry and topology, our work opens the door to artificially creating, customizing, and designing topological materials in rolled-up graphene-like systems.

cond-mat.mtrl-sci↗

arXiv2024

Driving noncollinear interlayer exchange coupling intrinsically in magnetic trilayers

Ferromagnetic side layers sandwiching a nonmagnetic spacer as a metallic trilayer has become a pivotal platform for achieving spintronic devices. Recent experiments demonstrate that manipulating the width or the nature of conducting spacer induces noncollinear magnetic alignment between the side layers. Our theoretical analysis reveals that altering the width of spacer significantly affects the interlayer exchange coupling (IEC), resulting in noncollinear alignment. Through analytic and first-principles methods, our study on the Fe/Ag/Fe trilayer shows that at a specific width of the Ag spacer, the magnetic moments of side layers tend to be perpendicular. This alignment is mediated by Ag quantum well states, exhibiting spin spirals across the trilayer. Our results reveal that the noncollinear IEC offers a degree of freedom to control magnetic devices and boot spintronic technology with improved transport capabilities.

cond-mat.mtrl-sci↗

arXiv2024

Large positive magnetoconductance in carbon nanoscrolls

We theoretically demonstrate that carbon nanoscrolls -- spirally wrapped graphene layers with open endpoints -- can be characterized by a large positive magnetoconductance. We show that when a carbon nanoscroll is subject to an axial magnetic field of several Tesla, the ballistic conductance at low carrier densities of the nanoscroll has an increase of about 200%. Importantly, we find that this positive magnetoconductance is not only preserved in an imperfect nanoscroll (with disorder or mild inter-turn misalignment) but can even be enhanced in the presence of on-site disorder. We prove that the positive magnetoconductance comes about the emergence of magnetic field-induced zero energy modes, specific of rolled-up geometries. Our results establish curved graphene systems as a new material platform displaying sizable magnetoresistive phenomena.

cond-mat.mes-hall↗

arXiv2024

On-demand higher-harmonic generation through nonlinear Hall effects in curved nanomembranes

The high-order Hall effects, which go beyond the ordinary, unlock more possibilities of electronic transport properties and functionalities. Pioneer works focus on the manufacture of complex nanostructures with low lattice symmetry to produce them. In this paper, we theoretically show that such high-order Hall effects can alternatively be generated by curving a conducting nanomembrane which is highly tunable and also enables anisotropy. Its Hall response can be tuned from first to fourth order by simply varying the direction and magnitude of the applied magnetic field. The dominant Hall current frequency can also be altered from zero to double, or even four times that of the applied alternating electric field. This phenomenon is critically dependent on the occurrence of high-order snake orbits associated with the effective magnetic-field dipoles and quadruples induced by the curved geometry. Our results offer pathways for spatially engineering magnetotransport, current rectification, and frequency multiplication in the bent conducting nanomembrane.

cond-mat.mes-hall↗

arXiv2024

Theta functions and adiabatic curvature on an Abelian variety

For an ample line bundle $L$ on an Abelian variety $M$, we study the theta functions associated with the family of line bundles $L\otimes T$ on $M$ indexed by $T\in \text{Pic}^{0}(M)$. Combined with an appropriate differential geometric setting, this leads to an explicit curvature computation of the direct image bundle $E$ on $\text{Pic}^{0}(M)$, whose fiber $E_{T}$ is the vector space spanned by the theta functions for the line bundle $L\otimes T$ on $M$. Some algebro-geometric properties of $E$ are also remarked.

math.AG↗

arXiv2021

Magnetoconductance modulations due to interlayer tunneling in radial superlattices

Radial superlattices are nanostructured materials obtained by rolling-up thin solid films into spiral-like tubular structures. The formation of these "high-order" superlattices from two-dimensional crystals or ultrathin films is expected to result in a transition of transport characteristics from two-dimensional to one-dimensional. Here, we show that a transport hallmark of radial superlattices is the appearance of magnetoconductance modulations in the presence of externally applied axial magnetic fields. This phenomenon critically relies on electronic interlayer tunneling processes that activates an unconventional Aharonov-Bohm-like effect. Using a combination of density functional theory calculations and low-energy continuum models, we determine the electronic states of a paradigmatic single-material radial superlattice -- a two-winding carbon nanoscroll -- and indeed show momentum-dependent oscillations of the magnetic states in axial configuration, which we demonstrate to be entirely due to hopping between the two windings of the spiral-shaped scroll.

cond-mat.mtrl-sci↗

arXiv2021

Resonant Nonlinear Hall Effect in Two-Dimensional Electron Systems

We study the Hall conductivity of a two-dimensional electron gas under an inhomogeneous magnetic field $B(x)$. First, we prove using the quantum kinetic theory that an odd magnetic field can lead to a purely nonlinear Hall response. Second, considering a real-space magnetic dipole consisting of a sign-changing magnetic field and based on numerical semiclassical dynamics, we unveil a parametric resonance involving the cyclotron ratio and a characteristic width of $B(x)$, which can greatly enhance the Hall response. Different from previous mechanisms that rely on the bulk Berry curvature dipole, here, the effect largely stems from boundary states associated with the real-space magnetic dipole. Our findings pave a new way to engineer current rectification and higher harmonic generation in two-dimensional materials having or not crystal inversion symmetry.

cond-mat.mes-hall↗

arXiv2021

Zero-magnetic-field Hall effects in artificially corrugated bilayer graphene

The ability to engineer the electronic band structure and, more strikingly, to access new exotic phase of matter has been the cornerstone of the advance of science and technology. Twisting van der Waals materials to form moiré superlattice is a powerful paradigm and can drive graphene from a normal metallic state into an insulating, superconducting, or ferromagnetic states. Here, we present a new route to create non-trivial band structure and consequently an exotic phase of matter via lithographically patterned strain (lattice deformation). This method is used to realize an artificially corrugated bilayer graphene wherein the real-space and momentum-space pseudo-magnetic fields (Berry curvatures) coexist and have nontrivial properties, namely, the Berry curvature dipole. This new class of condensed-matter systems enables us to observe the so-called nonlinear anomalous Hall effect and a new type of Hall effect without breaking the time-reversal symmetry. Such artificial material system and our approach to unconventional electronic states may open an avenue of geometrical and/or topological quantum phenomena as well as that of band engineering in van der Waals crystals.

cond-mat.mes-hall↗

arXiv2020

Topological Hall effect in single thick SrRuO3 layers induced by defect engineering

The topological Hall effect (THE) has been discovered in ultrathin SrRuO3 (SRO) films, where the interface between the SRO layer and another oxide layer breaks the inversion symmetry resulting in the appearance of THE. Thus, THE only occurs in ultra-thin SRO films of several unit cells. In addition to employing a heterostructure, the inversion symmetry can be broken intrinsically in bulk SRO by introducing defects. In this study THE is observed in 60 nm thick SRO films, in which defects and lattice distortions are introduced by helium ion irradiation. The irradiated SRO films exhibit a pronounced THE in a wide temperature range from 5 K to 80 K. These observations can be attributed to the emergence of Dzyaloshinskii-Moriya interaction as a result of artificial inversion symmetry breaking associated to the lattice defect engineering. The creation and control of the THE in oxide single layers can be realized by ex situ film processing. Therefore, this work provides new insights into the THE and illustrates a promising strategy to design novel spintronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Theta Functions and Adiabatic Curvature on a Torus

Let $M$ be a complex torus, $L_{\hatμ}\to M$ be positive line bundles parametrized by $\hat μ\in {\rm Pic}^0(M)$, and $E\to {\rm Pic}^0(M)$ be a vector bundle with $E|_{\hatμ}\cong H^0(M, L_{\hat μ})$. We endow the total family $\{L_{\hatμ}\}_{\hatμ}$ with a Hermitian metric that induces the $L^2$-metric on $H^0(M, L_{\hat μ})$ hence on $E$. By using theta functions $\{θ_m\}_{m}$ on $M\times M$ as a family of functions on the first factor $M$ with parameters in the second factor $M$, our computation of the full curvature tensor $Θ_E$ of $E$ with respect to this $L^2$-metric shows that $Θ_E$ is essentially an identity matrix multiplied by a constant $2$-form, which yields in particular the adiabatic curvature $c_1(E)$. After a natural base change $M\to \hat M$ so that $E\times_{\hat M} M:=E'$, we also obtain that $E'$ splits holomorphically into a direct sum of line bundles each of which is isomorphic to $L_{\hatμ=0}^*$. Physically, the spaces $H^0(M, L_{\hat μ})$ correspond to the lowest eigenvalue with respect to certain family of Hamiltonian operators on $M$ parametrized by $\hatμ$ or in physical notation, by wave vectors $\bf k$.

math.AG↗

arXiv2018

Defect-induced exchange bias in a single SrRuO3 layer

Exchange bias stems from the interaction between different magnetic phases and therefore it generally occurs in magnetic multilayers. Here we present a large exchange bias in a single SrRuO3 layer induced by helium ion irradiation. When the fluence increases, the induced defects not only suppress the magnetization and the Curie temperature, but also drive a metal-insulator transition at a low temperature. In particular, a large exchange bias field up to around 0.36 T can be created by the irradiation. This large exchange bias is related to the coexistence of different magnetic and structural phases that are introduced by embedded defects. Our work demonstrates that spintronic properties in complex oxides can be created and enhanced by applying ion irradiation.

cond-mat.mtrl-sci↗

arXiv2018

Tunable disorder and localization in the rare-earth nickelates

The rare-earth nickelates are a rich playground for transport properties, known to host non-Fermi liquid character, resistance saturation and metal-insulator transitions. We report a study of transport in LaNiO3 in the presence of tunable disorder induced by irradiation. While pristine LaNiO3 samples are metallic, highly irradiated samples show insulating behaviour at all temperatures. Using irradiation fluence as a tuning handle, we uncover an intermediate region hosting a metal-insulator transition. This transition falls within the Mott-Ioffe-Regel regime wherein the mean free path is comparable to lattice spacing. In the high temperature metallic regime, we find a transition from non-Fermi liquid to a Fermi-liquid-like character. On the insulating side of the metal-insulator transition, we find behaviour that is consistent with weak localization. This is reflected in magnetoresistance that scales with the square of the field and in resistivity. In the highly irradiated insulating samples, we find good agreement with variable range hopping, consistent with Anderson localization. We find qualitatively similar behaviour in thick PrNiO3 films as well. Our results demonstrate that ion irradiation can be used to tailor transport, serving as an excellent tool to study the physics of localization.

cond-mat.mtrl-sci↗

arXiv2018

Carrier driven antiferromagnetism and exchange-bias in SrRuO3/CaRuO3 heterostructures

Oxide heterostructures exhibit a rich variety of magnetic and transport properties which arise due to contact at an interface. This can lead to surprising effects that are very different from the bulk properties of the materials involved. We report the magnetic properties of bilayers of SrRuO3, a well known ferromagnet, and CaRuO3, which is nominally a paramagnet. We find intriguing features that are consistent with CaRuO3 developing dual magnetic character, with both a net moment as well as antiferromagnetic order. We argue the ordered SrRuO3 layer induces an undulating polarization profile in the conduction electrons of CaRuO3, by a mechanism akin to Friedel oscillations. At low temperatures, this oscillating polarization is inherited by rigid local moments within CaRuO3, leading to a robust exchange bias. We present ab initio simulations in support of this picture. Our results demonstrate a new ordering mechanism and throw light on the magnetic character of CaRuO3 .

cond-mat.mtrl-sci↗

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↗