arXiv subjects

Chun-Fu Chang

Publications and source records attributed to Chun-Fu Chang.

14 recordsLinked to original sources

arXiv2026

Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide

High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in two ferrimagnetic spinel-structured HEOs with compositions (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and (Cr,Mn,Fe,Co,Ni)$_{2.4}$Ga$_{0.6}$O$_4$. We find that while the magnetic transition is simultaneous for all chemical species, the rate at which their magnetic moments grow is strongly cation dependent. This behavior is explained by the varying $\textit{3d}$ crystal field level fillings of the magnetic cations, which in turn determine their ability to participate in the different magnetic exchange pathways available in the spinel structure. Dominant $A$-$B$ sublattice exchange enables some species to harden rapidly ($\textit{e.g.}$ tetrahedral Fe$^{3+}$ and octahedral Ni$^{2+}$) while others exhibit a sluggish transition due to frustration from competing interactions ($\textit{e.g.}$ octahedral Fe$^{3+}$ and Cr$^{3+}$). Non-magnetic substitution suppresses these differences, introducing broken magnetic linkages that relieve frustration. Tailoring the magnetism of HEO spinels therefore requires detailed knowledge of both their site selectivities and their exchange pathways.

cond-mat.mtrl-sci↗

arXiv2026

Nitrogen doping induced metal-insulator transition with iso-symmetric character in rutile VO2

Metal-insulator transitions (MITs) in correlated oxides offer immense potential for next-generation Mottronic devices. However, their integration into practical applications is often hindered by the coupling of MITs with symmetry-lowering structural phase transitions, which limits switching speed and endurance. In this study, we engineered an iso-symmetric MIT on average in epitaxial rutile VO2 thin films via an in-situ nitrogen doping strategy. Nitrogen incorporation effectively suppresses V-V dimerization, enabling an iso-symmetric MIT, while preserving the original crystal symmetry. Furthermore, in-operando time-resolved optical reflectivity measurements revealed a shortened switching time in nitrogen-doped films, highlighting their enhanced performance. Our findings provide critical insights into the underlying mechanisms of MITs and introduce anion doping as a powerful tool for tailoring phase transitions in strongly correlated electron systems. This approach opens new avenues for the development of high-performance electronic and photonic devices.

cond-mat.mtrl-sci↗

arXiv2026

UCd$_{11}$: A strongly localized 5$f^3$ material

UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (PES) data of UCd$_{11}$ reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd$_{11}$, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd$_{11}$ is a highly localized uranium 5$f^3$ system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5$f$ behavior.

cond-mat.str-el↗

arXiv2024

Large Enhancement of Properties in Strained Lead-free Multiferroic Solid Solutions with Strong Deviation from Vegard's Law

Efforts to combine the advantages of multiple systems to enhance functionlities through solid solution design present a great challenge due to the constraint imposed by the classical Vegard law. Here, we successfully navigate this trade off by leveraging the synergistic effect of chemical doping and strain engineering in solid solution system of BiFeO3 BaTiO3. Unlike bulks, a significant deviation from the Vegard law accompanying with enhanced multiferroism is observed in the strained solid solution epitaxial films, where we achieve a pronounced tetragonality, enhanced saturated magnetization, substantial polarization, high ferroelectric Curie temperature, all while maintaining impressively low leakage current. These characteristics surpass the properties of their parent BiFeO3 and BaTiO3 films. Moreover, the superior ferroelectricity has never been reported in corresponding bulks. These findings underscore the potential of strained BiFeO3 BaTiO3 films as lead-free, room-temperature multiferroics.

cond-mat.mtrl-sci↗

arXiv2024

Quantifying the U $5f$ covalence and degree of localization in U intermetallics

A procedure for quantifying the U $5f$ electrons' covalence and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross-sections allows the disentanglement of the U\,$5f$ contribution to the valence band from the various other atomic subshells so that the computational parameters in the DFT\,+\,DMFT can be reliably determined. Applying this method to UGa$_2$ and UB$_2$ as model compounds from opposite ends of the (de)localization range, we have achieved excellent simulations of the valence band and core-level spectra. The width in the distribution of atomic U\,$5f$ configurations contributing to the ground state, as obtained from the calculations, quantifies the correlated nature and degree of localization of the U\,5$f$. The findings permit answering the longstanding question why different spectroscopic techniques give seemingly different numbers for the U 5$f$ valence in intermetallic U compounds.

cond-mat.str-el↗

arXiv2024

Enhanced polarization switching characteristics of HfO2 ultrathin films via acceptor-donor co-doping

In the realm of ferroelectric memories, HfO2-based ferroelectrics stand out because of their exceptional CMOS compatibility and scalability. Nevertheless, their switchable polarization and switching speed are not on par with those of perovskite ferroelectrics. It is widely acknowledged that defects play a crucial role in stabilizing the metastable polar phase of HfO2. Simultaneously, defects also pin the domain walls and impede the switching process, ultimately rendering the sluggish switching of HfO2. Herein, we present an effective strategy involving acceptor-donor co-doping to effectively tackle this dilemma. Remarkably enhanced ferroelectricity and the fastest switching process ever reported among HfO2 polar devices are observed in La3+-Ta5+ co-doped HfO2 ultrathin films. Moreover, robust macro-electrical characteristics of co-doped films persist even at a thickness as low as 3 nm, expanding potential applications of HfO2 in ultrathin devices. Our systematic investigations further demonstrate that synergistic effects of uniform microstructure and smaller switching barrier introduced by co-doping ensure the enhanced ferroelectricity and shortened switching time. The co-doping strategy offers an effective avenue to control the defect state and improve the ferroelectric properties of HfO2 films.

cond-mat.mtrl-sci↗

arXiv2023

Fe substitution in URu$_2$Si$_2$: singlet magnetism in an extended Doniach phase diagram

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ from $x$\,=\,0.0 to 2.0 by U\,4$f$ core-level photoelectron spectroscopy and have observed non-monotonic changes in the spectra. The initial increase and subsequent decrease of the spectral weight of the 4$f$ core level satellite with increasing $x$ stands for a non-monotonic 5$f$ filling across the substitution series. The competition of chemical pressure and increase of the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such a behavior. An extended Doniach phase diagram including the $x$ dependence of the density of states is proposed. Also in URu$_{2-x}$Fe$_x$Si$_2$ the ground state is a singlet or quasi-doublet state consisting of two singlets. Hence, the formation of magnetic order in the URu$_{2-x}$Fe$_x$Si$_2$ substitution series must be explained within a singlet magnetism model.

cond-mat.str-el↗

arXiv2023

Singlet magnetism in intermetallic UGa$_2$ unveiled by inelastic x-ray scattering

Using high resolution tender-x-ray resonant inelastic scattering and hard-x-ray non-resonant inelastic scattering beyond the dipole limit we were able to detect electronic excitations in intermetallic UGa$_2$ that are highly atomic in nature. Analysis of the spectral lineshape reveals that the local $5f^2$ configuration characterizes the correlated nature of this ferromagnet. The orientation and directional dependence of the spectra indicate that the ground state is made of the $\Gamma_1$ singlet and/or $\Gamma_6$ doublet symmetry. With the ordered moment in the $ab$ plane, we infer that the magnetism originates from the higher lying $\Gamma_6$ doublet being mixed with the $\Gamma_1$ singlet due to inter-site exchange, qualifying UGa$_2$ to be a true quantum magnet. The ability to observe atomic excitations is crucial to resolve the on-going debate about the degree of localization versus itineracy in U intermetallics.

cond-mat.str-el↗

arXiv2023

Hidden Hydroxides in KOH-Grown BaNiO3 Crystals: A Potential Link to Their Catalytic Behavior

The hexagonal perovskite BaNiO3, prepared via non-ceramic approaches, is known to act as a good catalyst for the oxygen-evolution reaction (OER) in alkaline media. Here we report our observation that BaNiO3 synthesized via KOH flux growth and high O2 pressure ceramic synthesis have different magnetic properties. We show that this is because the KOH flux-grown crystals made in open-air are actually a hydroxide-containing form of BaNiO3 that can be dried upon annealing in O2 flow. This work not only unveils a previously unknown aspect of the BaNiO3 OER catalyst and offers some insights into the underlying mechanism, but also suggests that hydroxide ions may be present in other hexagonal perovskite oxides prepared in wet conditions.

cond-mat.mtrl-sci↗

arXiv2023

Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts have been devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, we have systematically investigated the spin state in epitaxial LaCoO3 thin films to clarify the mechanism of strain induced ferromagnetism using element-specific x-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, we unambiguously demonstrate that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) are practically a low spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high spin and low spin states with a ratio close to 1:3. From the identification of this spin state ratio, we infer that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films.

cond-mat.mtrl-sci↗

arXiv2012

Mimicking the Standard Model Higgs Boson in UMSSM

Motivated by the recent results in the standard model (SM) Higgs boson search at the Large Hadron Collider (LHC) we investigate the SM-like CP-even Higgs boson of the U(1)'-extended minimal supersymmetric standard model (UMSSM) and its branching ratio into the b b-bar, W W*, and χ^0_1 χ^0_1 modes. In the Summer 2011, a 2 sigma excess was reported in the channel H -> W W* -> l+ nu l- nu-bar around 130 - 140 GeV range. Later on in December 2011 announcements were made that an excess was seen in the 124-126 GeV range, while the SM Higgs boson above 131 GeV up to about 600 GeV is ruled out. We examine two scenarios of these mass ranges: (i) 130 GeV < M_{h_{SM-like}} < 141 GeV and show that the Higgs boson can decay into invisible neutralinos to evade the SM bound; and (ii) 120 GeV < M_{h_{\rm SM-like}} < 130 GeV and show that the Higgs boson can avoid decaying into neutralinos and thus gives enhanced rates into visible particles. We use the ηmodel of E_6 with TeV scale supersymmetry to illustrate the idea by scanning the parameter space to realize these two different scenarios.

hep-ph↗

arXiv2011

Supersymmetric Decays of the Z' Boson

The decay of the Z' boson into supersymmetric particles is studied. We investigate how these supersymmetric modes affect the current limits from the Tevatron and project the expected sensitivities at the LHC. Employing three representative supersymmetric Z' models, namely, E_6, U(1)_{B-L}, and the sequential model, we show that the current limits of the Z' mass from the Tevatron could be reduced substantially due to the weakening of the branching ratio into leptonic pairs. The mass reach for the E_6 Z' bosons is about 1.3-1.5 TeV at the LHC-7 (1 fb^{-1}), about 2.5 - 2.6 TeV at the LHC-10 (10 fb^{-1}), and about 4.2 - 4.3 TeV at the LHC-14 (100 fb^{-1}). A similar mass reach for the U(1)_{B-L} Z' is also obtained. We also examine the potential of identifying various supersymmetric decay modes of the Z' boson because it may play a crucial role in the detailed dynamics of supersymmetry breaking.

hep-ph↗

arXiv2011

Global Study of Electron-Quark Unparticle Interactions

We perform a global fit on parity-conserving electron-quark interactions via spin-1 unparticle exchange. Besides the peculiar features of unparticle exchange due to non-integral values for the scaling dimension d_U and a non-trivial phase factor exp (-id_U π) associated with a time-like unparticle propagator, the energy dependence s^{-2 + d_U} of the unparticle contributions in the scattering amplitudes are also taken into account. The high energy data sets taken into consideration in our analysis are from (1) deep inelastic scattering at high Q^2 from ZEUS and H1, (2) Drell-Yan production at Run II of CDF and DO, and (3) e^+e^- --> hadrons at LEPII. The hadronic data at LEPII by itself indicated a 3-4 sigma preference of new physics over the Standard Model. However, when all data sets are combined, no preference for unparticle effects can be given. We thus deduce an improved 95% confidence level limit on the unparticle energy scale Λ_\U.

hep-ph↗

arXiv2008

Unparticle effects in photon-photon scattering

Elastic photon-photon scattering can only occur via loop diagrams in the standard model and is naturally suppressed. Unparticle can induce tree-level photon-photon scattering through the operator F_{μν} F^{μν} O_\U for spin-0 unparticle or F_{μα} F^α_ν O^{μν}_\U for spin-2 unparticle. Due to the peculiar CP-conserving phase \exp(-i d_\U π) associated with the s-channel unparticle propagator, its interference effects with the t- and u-channels on the total cross section and the angular distribution are found to be some significance. In addition, we show that the cross sections via unparticle exchange can be substantially larger than the standard model contribution.

hep-ph↗