Several research studies have investigated the degradation of BaTiO3-based dielectric capacitor materials, focusing on the impact of composition, defect chemistry, and microstructural design to limit the electromigration of oxygen vacancies under electric fields at finite temperatures. Electromigration can be a dominant mechanism that controls failure rates in the individual multilayer ceramic capacitor (MLCC) components in testing the reliability of failures with highly accelerated lifetime testing (HALT) to determine the mean time to failure of MLCCs surface mounted onto printed circuit boards (PCBs). Conventional assumptions often consider these failures as independent, with no interaction between components on the PCB. However, this study employs a Physics of Failure (PoF) approach to closely examine transient degradation and its impact on MLCC reliability, emphasizing thermal crosstalk and its influence on dependent and independent failure rates. Finite element analysis thermal modeling and infrared thermography were used to assess the impact of circuit layout and component spacing on heat dissipation and thermal crosstalk under various electrical stress conditions. The study distinguishes between dependent and independent failures under a HALT, quantified through a β′ factor reflecting common cause failures due to thermal crosstalk. Through a series of experimental and statistical analyses, the β′ factor is evaluated with respect to temperature, voltage, and component spacing. These insights highlight the importance of understanding the nature of the data in reliability testing of MLCCs and optimizing the layout design of high-density circuits to mitigate dependent failures, improving overall reliability and informing better design and packaging strategies.

The charge density wave in (TaSe4)2I has drawn much attention recently as a controversial candidate for an axion insulator where the CDW breaks the chiral symmetry of the Weyl semimetal. Here we use ultrafast x-ray scattering to study the collective modes of this CDW. By measuring several diffraction peaks we find that the order parameter involves coupled optical and acoustic modes. For strong near-infrared excitation, the dynamics of the x-ray diffraction show evidence of photoinduced inversion of both components of the CDW order parameter, and associated domain walls. These results demonstrate the potential of ultrafast methods to induce topological defects through highly nonequilibrium dynamics. In (TaSe4)2I these defects should lead to exotic electronic states due to the nontrivial topology of the band structure.

Ag3AuSe2 and Ag3AuTe2 were previously predicted to be narrow direct gap semiconductors with the same chiral structure type. Recent computational studies using Perdew-Burke-Ernzerhof (PBE) functional highlighted their potential band gap tunability via strain application. For example, Ag3AuSe2 was predicted to exhibit full band closure above 4% tensile strain. In this study, we explored chemical substitution to examine the density functional theory (DFT) predictions, by replacing Se2− with larger Te2− anions. We synthesized and characterized the electronic and optical properties of Ag3Au[Se(1−x)Te(x)]2 solid solutions for x from 0 to 1. Our findings revealed that the lattice constants increase linearly with Te incorporation, reaching 3.6% expansion at 90% Se2− to Te2− substitution. The activation energy and optical band gap of Ag3Au(Se,Te)2 were determined using electrical resistivity and ultraviolet–visible (UV-Vis) diffuse reflectance measurements. The band gap decreased with increasing Te content, although hybrid functionals are necessary to correctly predict the gap. Further computational studies on the band structures of Ag3Au(Se,Te)2 alloys would shed light on the impact of lattice parameter modification via chemical substitution on band gap tunability.

Previous studies on natural samples of pampaloite (AuSbTe) revealed the crystal structure of a potentially cleavable and/or exfoliable material, while studies on natural and synthetic montbrayite (Sb-containing Au2Te3) claimed various chemical compositions for this low-symmetry compound. Few investigations of synthetic samples have been reported for both materials, leaving much of their chemical, thermal, and electronic characteristics unknown. Here, we investigate the stability, electronic properties, and synthesis of the gold antimony tellurides AuSbTe and Au1.9Sb0.46Te2.64 (montbrayite). Differential thermal analysis and in situ powder X-ray diffraction revealed that AuSbTe is incongruently melting, while Au1.9Sb0.46Te2.64 is congruently melting. Calculations of the band structures and four-point resistivity measurements showed that AuSbTe is a semiconductor and Au1.9Sb0.46Te2.64 a metal. Various synthesis attempts confirmed the limited stable chemical composition of Au1.9Sb0.46Te2.64, identified successful methods to synthesize both compounds, and highlighted the challenges associated with single-crystal synthesis of AuSbTe.

The intertwining between two ordered states that arise from the same interactions is reflected in the dynamics of their coupled collective excitations. While the equilibrium phase diagram resulting from such intertwined orders has been extensively studied, the dynamic competition between non-equilibrium modes is a largely unexplored territory. Here, we introduce a multimodal STM-based pump-probe technique, that combines ultrafast tunneling microscopy (USTM), ultrafast point-contact spectroscopy (UPC), and optical pump-probe reflectance (OPPR) on femtosecond timescale, all within a single instrument. Using this platform, we investigate the collective excitations of the unconventional charge density wave insulator (TaSe4)2I. Our UPC measurements reveal charge oscillations at 0.22 THz, with a temperature dependence that matches the theoretically predicted behavior of the long-sought massive phason gaining mass through the Anderson-Higgs mechanism. Unexpectedly, the data also reveals a second mode at 0.11 THz exhibiting similar temperature and polarization dependence with comparable mode intensity. These features, along with the robust 1/2 frequency ratio locking suggest that the 0.11 THz phason is a 'daughter mode' that arises from the splitting of the 0.22 THz massive phason into two massless phasons via parametric amplification, analogous to the decay of a neutral pion into two photons. Strikingly, comparison with OPPR data reveals that the daughter phason competes with and suppresses the amplitudon at proximate frequency. Our studies reveal an unexplored mechanism for the generation and extinction of collective excitations in quantum materials and pave the way for a microscopic understanding of ultrafast phenomena.

AlN/GaN/AlN high electron mobility transistors (HEMTs) offer enhanced carrier confinement and higher breakdown voltage than conventional AlGaN/GaN HEMTs. In this work, Raman thermometry was used to characterize the self-heating behavior of a single-finger AlN/GaN/AlN HEMT on 6H-SiC. A 3D finite element analysis model was created to optimize the thermal design of the device structure. Simulation results reveal that the optimal buffer layer thicknesses to minimize the channel temperature rise of AlN/GaN/AlN HEMTs on 6H-SiC and diamond substrates are similar to 2 mu m and similar to 0.7 mu m, respectively. Moreover, diamond substrate integration further enhances the thermal performance, achieving a similar to 45% and similar to 53% reduction in the device thermal resistance as compared to those of an AlN/GaN/AlN HEMT on 6H-SiC and an AlGaN/GaN HEMT on 4H-SiC, respectively.

Rare-earth emitters in solids are well-suited for implementing efficient, long-lived quantum memory coupled to integrated photonics for scalable quantum technologies. They are typically introduced as dopants in a solid-state host, but this introduces disorder and limits the available density of emitters. Stoichiometric materials can offer high densities with narrow optical linewidths. The regular spacing of emitters also opens possibilities for quantum information processing and collective effects. Here we show narrow optical linewidths in a layered stoichiometric crystalline material, NaEu(IO_3)_4. We observed an inhomogeneous linewidth of 2.2(1) GHz and a homogeneous linewidth of 120(4) kHz. Using spectral hole-burning techniques, we observe a hyperfine spin lifetime of 1.9(4) s. Furthermore, we demonstrate an atomic frequency comb delay of up to 800 ns.

Gallium nitride (GaN)-based radio frequency (RF) power amplifiers are spearheading the deployment of next-generation wireless systems owing to the large power handling capability at high frequencies and high-power-added efficiency. Unfortunately, this high power density operation leads to severe overheating, which reduces its lifetime and efficiency. Thus, correctly characterizing the temperature rise is of crucial importance to properly design GaN devices and cooling solutions. Optical-based thermometry techniques such as Raman thermometry and infrared (IR) thermography are commonly used to estimate the peak temperature rise, but they are limited by optical access, topside metallization, and depth averaging. Gate resistance thermometry (GRT) offers an alternative method to measure the temperature without needing optical access to the channel. Therefore, in this work, Raman thermometry is used in conjunction with GRT and electrothermal modeling to determine the accuracy of each method for a field-plated GaN high electron mobility transistor (HEMT) under various bias conditions. While both Raman thermometry and GRT measured a similar temperature rise under fully open (FO) channel conditions, it was found that GRT was better at estimating the peak temperature under a partially pinched-off (PPO) bias condition due to the source-connected field plate (SCFP) restricting optical access to the drain side of the gate edge.

In a system of charged chiral fermions driven out of equilibrium, an electric current parallel to the magnetic field can generate a dynamic instability by which electromagnetic waves become amplified. Whether a similar instability can occur in chiral solid-state systems remains an open question. Using time-domain terahertz (THz) emission spectroscopy, we detect signatures of what we dub a ``dynamic magneto-chiral instability" in elemental tellurium, a structurally chiral crystal. Upon transient photoexcitation in a moderate external magnetic field, tellurium emits THz radiation consisting of coherent modes that amplify over time. An explanation for this amplification is proposed using a theoretical model based on a dynamic instability of electromagnetic waves interacting with infrared-active oscillators of impurity acceptor states in tellurium to form an amplifying polariton. Our work not only uncovers the presence of a magneto-chiral instability but also highlights its promise for THz-wave amplification in chiral materials.

The quasi-one-dimensional charge density wave (CDW) material (TaSe4)2I has been recently predicted to host Kramers-Weyl (KW) fermions which should exist in the vicinity of high symmetry points in the Brillouin zone in chiral materials with strong spin-orbit coupling. However, direct spectroscopic evidence of KW fermions is limited. Here we use helicity-dependent laser-based angle-resolved photoemission spectroscopy (ARPES) in conjunction with tight-binding and first-principles calculations to identify KW fermions in (TaSe4)2I. We find that topological and symmetry considerations place distinct constraints on the (pseudo-) spin texture and the observed spectra around a KW node. Our findings highlight the unique topological nature of (TaSe4)2I and provide a pathway for identifying KW fermions in other chiral materials. It has been predicted that the quasi-one-dimensional charge density wave material (TaSe4)2I hosts Kramers-Weyl fermions, but direct spectroscopic evidence of this is limited. Here, ARPES and theoretical calculations reveal signatures that may indicate the presence of Kramers-Weyl fermions.

The interaction of weak light with correlated Weyl semimetals (WSMs) provides a unique platform to explore nonequilibrium phases close to the ground state where a fragile balance of competing interactions, correlations, and chirality stabilizes the topological state. In this study, we probe (TaSe4)2I, a prototypical correlated WSM, under very weak optical pumping using the circular photogalvanic effect and Raman spectroscopy. Surprisingly, we find that there is strong suppression of chiral response in (TaSe4)2I under very low optical excitation power. This loss of chirality is attributed to an optically driven reversible topological phase transition into an achiral state distinct from the ground state. The microscopic mechanism of this transformation is supported by fluencedependent Raman spectra, which reveal a new peak at low pump fluences that disappears above the threshold fluence suggesting the formation of symmetric TaSe4 structures. This all-optical manipulation of topology at very low powers showcases a way to control the properties of topological materials with strong electron-electron and electron-phonon coupling that are not driven far away from their ground states.

Insulators containing chains of magnetic transition metal cations provide platforms for probing spin-12 dynamics and quantum critical behavior. Li2CoCl4 contains edge-sharing CoCl6 octahedra that form chains along the crystallographic c axis and orders antiferromagnetically at zero field, but questions remain about its applied-field magnetic structure and the Co2+ spin state. Here, we show with neutron diffraction on a polycrystalline sample how the antialigned chains of cobalt moments begin to transition to a ferromagnetic state above 1.6 T. Further, using magnetic resonance absorption measurements and noninteracting spin models, we reveal the strongly anisotropic nature of the Co2+ion's XY-like magnetic behavior (g1 = 2.77 and gl = 5.23) and its J = 12 ground state. We therefore supply the magnetic structures and anisotropic description needed to explore the dynamics of the field-driven magnetic phases, laying the foundation for further experimental and theoretical studies.

alpha-Ga2O3 is one of the ultra-wide bandgap semiconductors that gives promise to the development of next-generation power electronics. However, due to its thermodynamic instability and low thermal conductivity, overheating concerns hinder the deployment of alpha-Ga2O3 devices. This study reveals the detrimental impact of thermal crosstalk in multi-finger alpha-Ga2O3 MOSFETs. The thermal conductivity of alpha-Ga2O3 (similar to 12 W/m.K at room temperature) was determined via first-principles calculations and laser-based pump-probe measurements. Device thermal characterization and modeling were performed to design a vertebrae-shaped multi-finger device layout that mitigates thermal crosstalk by decentralizing the overall device heat generation profile.

Coupling Weyl quasiparticles and charge density waves (CDWs) can lead to fascinating band renormalization and many-body effects beyond band folding and Peierls gaps. For the quasi-one-dimensional chiral compound (TaSe4)2I with an incommensurate CDW transition at TC = 263 K, photoemission mappings thus far are intriguing due to suppressed emission near the Fermi level. Models for this unconventional behavior include axion insulator phases, correlation pseudogaps, polaron subbands, bipolaron bound states, etc. Our photoemission measurements show sharp quasiparticle bands crossing the Fermi level at T > TC, but for T < TC, these bands retain their dispersions with no Peierls or axion gaps at the Weyl points. Instead, occupied band edges recede from the Fermi level, opening a spectral gap. Our results confirm localization of quasiparticles (holes created by photoemission) is the key physics, which suppresses spectral weights over an energy window governed by incommensurate modulation and inherent phase defects of CDW.

Abstract Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- ( $${{{{{{{\mathcal{T}}}}}}}}$$ T -) invariant (helical) 3D TCIs—termed higher-order TCIs (HOTIs)—the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the intrinsic bulk topological properties of spinful 3D insulators. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), “spin-Weyl” semimetals, and $${{{{{{{\mathcal{T}}}}}}}}$$ T -doubled axion insulator (T-DAXI) states with nontrivial partial axion angles indicative of a 3D spin-magnetoelectric bulk response and half-quantized 2D TI surface states originating from a partial parity anomaly. Using ab-initio calculations, we demonstrate that β-MoTe2 realizes a spin-Weyl state and that α-BiBr hosts both 3D QSHI and T-DAXI regimes.

Boron (B)-substituted wurtzite AlN (Al1-x B x N) is a recently discovered wurtzite ferroelectric material that offers several advantages over ferroelectric Hf1-x Zr x O2 and PbZr1-x Ti x O3. Such benefits include a relatively low growth temperature as well as a thermally stable, and thickness-stable ferroelectric polarization; these factors are promising for the development of ferroelectric nonvolatile random-access memory (FeRAM) that are CMOS-compatible, scalable, and reliable for storing data in harsh environments. However, wurtzite ferroelectric materials may undergo exacerbated self-heating upon polarization switching relative to other ferroelectric materials; the larger energy loss is anticipated due to the higher coercive field and remanent polarization. This work provides insight into the polarization switching-induced self-heating of future FeRAM based on Al1-x B x N. It was experimentally observed that the thermal conductivity of Al1-x B x N thin films drops from 40.9 W m-1 K-1 to 4.35 W m-1 K-1 (which is 2 orders of magnitude lower than that of bulk AlN) when the B composition (x) increases from 0 to 0.18. The transient thermal response of an Al0.93B0.07N metal-ferroelectric-metal (MFM) capacitor was investigated using micro-Raman thermometry and validated via device thermal modeling. Further simulation studies reveal that the large heat generation rate and the low thermal conductivity is predicted to induce an instantaneous temperature rise that may exceed 150 degrees C in a FeRAM device based on a 5 nm thick Al1-x B x N film at GHz frequency switching. In addition, thermal crosstalk within a FeRAM cell array exacerbates the self-heating, resulting in a predicted steady-state temperature rise that is an order of magnitude higher than that of a single bit-cell.

The quasi-one-dimensional chiral compound (TaSe4)2I has been extensively studied as a prime example of a topological Weyl semimetal. Upon crossing its phase transition temperature TCDW approximate to 263 K, (TaSe4)2I exhibits incommensurate charge density wave (CDW) modulations described by the well-defined propagation vector similar to(0.05, 0.05, 0.11), oblique to the TaSe4 chains. Although optical and transport properties greatly depend on chirality, there is no systematic report about chiral domain size for (TaSe4)2I. In this study, our single-crystal scattering refinements reveal a bulk iodine deficiency, and Flack parameter measurements on multiple crystals demonstrate that separate (TaSe4)2I crystals have uniform handedness, supported by direct imaging and helicitydependent terahertz emission spectroscopy. Our single-crystal x-ray scattering and calculated diffraction patterns identify multiple diffuse features and create a real-space picture of the temperature-dependent (TaSe4)2I crystal structure. The short-range diffuse features are present at room temperature and decrease in intensity as the CDW modulation develops. These transverse displacements, along with electron pinning from the iodine deficiency, help explain why (TaSe4)2I behaves as an electronic semiconductor at temperatures above and below TCDW, despite a metallic band structure calculated from density functional theory of the ideal structure.

Recent band structure calculations have suggested the potential for band tuning in the chiral semiconductor Ag3AuTe2 to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag3AuTe2 and investigate its transport and optical properties and mechanical compressibility. Transport measurements reveal the semiconducting behavior of Ag3AuTe2 with high resistivity and an activation energy E-a of 0.2 eV. The optical bandgap determined by diffuse reflectance measurements is about three times wider than the experimental E-a. Despite the difference, both experimental gaps fall within the range of predicted bandgaps by our first-principles density functional theory (DFT) calculations employing the Perdew-Burke-Ernzerhof and modified Becke-Johnson methods. Furthermore, our DFT simulations predict a progressive narrowing of the bandgap under compressive strain, with a full closure expected at a strain of -4% relative to the lattice parameter. To evaluate the feasibility of gap tunability at such substantial strain, the high-pressure behavior of Ag3AuTe2 was investigated by in situ high-pressure x-ray diffraction up to 47 GPa. Mechanical compression beyond 4% resulted in a pressure-induced structural transformation, indicating the possibility of substantial gap modulation under extreme compression conditions.

We present the characterization of stoichiometric europium materials as progress towards realizing long-lived, ensemble-based quantum memory devices. The candidate materials demonstrate inhomogeneous optical linewidths similar to those in doped materials with 1000x larger densities.

As the field of exfoliated van der Waals electronics grows to include complex heterostructures, the variety of available in-plane symmetries and geometries becomes increasingly valuable. In this work, we present an efficient chemical vapor transport synthesis of NbSe2I2 with triclinic space group P-1. This material contains Nb–Nb dimers and an inplane crystallographic angle γ = 61.3◦. We show that NbSe2I2 can be exfoliated down to few-layer and mono-layer structures and used Raman spectroscopy to test the preservation of crystal structure of exfoliated thin films. The crystal structure was verified by singlecrystal and powder X-ray diffraction. Density functional theory calculations show triclinic NbSe2I2 to be a semiconductor with a band gap of around 1 eV, with similar band structure features for bulk and mono-layer crystals. The physical properties of NbSe2I2 have been characterized by transport, thermal, optical, and magnetic measurements, demonstrating triclinic NbSe2I2 to be a diamagnetic semiconductor that does not exhibit any phase transformation below room temperature.