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1. Unveiling the Nanoscale Dielectric Gap and Its Influence on Ferroelectric Polarization Switching in Scanning Probe Microscopy

   https://doi.org/10.1002/adfm.202406944  

   Eom, Seongmun; Kavle, Pravin; Kang, Deokyoung; Kim, Yeongyu; Martin, Lane W; Hong, Seungbum (March 2025, Advanced Functional Materials)

   Abstract The dielectric gap between the scanning probe microscopy (SPM) tip and the surface of a ferroelectric using conductive atomic force microscopy and piezoresponse force microscopy (PFM) is investigated. While the gap functions as a dielectric layer, it also allows tunneling current to inject charges into the ferroelectric when a critical loading force between 10–20 µN is applied to a tip with a radius of 25 nm under a bias voltage of 0.5 V. It is observed that the permittivity of the dielectric gap determines the coercive voltage measured by the piezoresponse hysteresis loop. While such studies done in air often produce coercive voltages much larger than those studied for the same materials in capacitor‐based studies, the use of high permittivity media such as water (ɛ_r= 79) or silicone oil (ɛ_r= 2.1‐2.8) produces coercive fields that more closely match those measured in conventional capacitor‐based polarization hysteresis loop measurements. Furthermore, using water as a dielectric medium in PFM imaging enhances the accuracy in extracting the amplitude and phase data from periodically poled lithium niobate crystals. These findings provide insight into the nanoscale phenomena of polarization switching instigated by the SPM tip and provide a pathway to improved quantitative studies. 

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2. Threshold voltage control with high-temperature gate-oxide annealing in ultrawide bandgap AlGaN-channel MOSHFETs

   https://doi.org/10.35848/1882-0786/ac8bc4  

   Mollah, Shahab; Hussain, Kamal; Mamun, Abdullah; Alam, Md Didarul; Chandrashekhar, MVS; Simin, Grigory; Khan, Asif (September 2022, Applied Physics Express)

   Abstract We report threshold voltage (V_TH) control in ultrawide bandgap Al_0.4Ga_0.6N-channel metal oxide semiconductor heterostructure field-effect transistors using a high-temperature (300 °C) anneal of the high-kZrO₂gate-insulator. Annealing switched the polarity of the fixed charges at the ZrO₂/AlGaN interface from +5.5 × 10¹³cm⁻²to −4.2 × 10¹³cm⁻², pinningV_THat ∼ (−12 V), reducing gate leakage by ∼10³, and improving subthreshold swing 2× (116 mV decade⁻¹). It also enabled the gate to repeatedly withstand voltages from −40 to +18 V, allowing the channel to be overdriven doubling the peak currents to ∼0.5 A mm⁻¹. 

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3. Exploring dark forces with multimessenger studies of extreme mass ratio inspirals

   https://doi.org/10.1088/1475-7516/2024/09/023  

   Bhalla, Badal; Sinha, Kuver; Xu, Tao (September 2024, Journal of Cosmology and Astroparticle Physics)

   Abstract The exploration of dark sector interactions via gravitational waves (GWs) from binary inspirals has been a subject of recent interest. We study dark forces using extreme mass ratio inspirals (EMRIs), pointing out two issues of interest. Firstly, the innermost stable circular orbit (ISCO) of the EMRI, which sets the characteristic length scale of the system and hence the dark force range to which it exhibits enhanced sensitivity, probes force mediator masses that complement those studied with supermassive black hole (SMBH) or neutron star binaries. The LISA mission (the proposedμAres detector) will probe mediators with massesm_V∼ 10^-16  eV (m_V∼ 10^-18  eV), corresponding to ISCOs of 10⁶M_⊙(10⁸M_⊙) central SMBHs. Secondly, while the sensitivity to dark couplings is typically limited by the uncertainty in the binary component masses, independent mass measurements of the central SMBH through reverberation mapping campaigns or the motion of dynamical tracers enable one to break this degeneracy. Our results therefore highlight the necessity for coordinated studies, loosely referred to as “multimessenger”, between futureμHz- mHz GW observatories and ongoing and forthcoming SMBH mass measurement campaigns, including OzDES-RM, SDSS-RM, and SDSS-V Black Hole Mapper. 

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4. Polarization near dislocation cores in SrTiO3 single crystals: The role of flexoelectricity

   https://doi.org/10.1063/5.0202453  

   Cheng, Xiaoxing; Wang, Bo; Chen, Long-Qing (March 2024, Applied Physics Letters)

   Spontaneous polarization as large as ∼28 μC/cm2 was recently observed around the dislocation cores in non-polar SrTiO3 bulk crystals, and its origin was attributed to the flexoelectric effect, i.e., polarization induced by strain gradients. However, the roles of flexoelectricity, relative to other electromechanical contributions, and the nature of dislocations, i.e., edge vs screw dislocations in the induced polarization, are not well understood. In this work, we study the role of flexoelectricity in inducing polarization around three types of dislocation cores in SrTiO3: b=a(100) edge dislocation, b=a(110) edge dislocation, and b=a(010) screw dislocation, where b is the Burgers vector. For the edge dislocations, polarization can be induced by electrostriction alone, while flexoelectricity is essential for stabilizing the symmetric polarization pattern. The shear component of the flexoelectric tensor has a dominant effect on the magnitude and spatial distribution of the flexoelectric polarization. In contrast, no polarization is induced around the b=a(010) screw dislocation through either electrostriction or flexoelectricity. Our findings provide an in-depth understanding of the role of flexoelectricity in inducing polarization around dislocation cores and offer insights into the defect engineering of dielectric/ferroelectric materials. 

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5. The Cool Circumgalactic Medium of Low-redshift Star-forming Galaxies. I. Empirical Model and Mean Properties

   https://doi.org/10.3847/1538-4357/acf217  

   Faerman, Yakov; Werk, Jessica K (October 2023, The Astrophysical Journal)

   Abstract We present an analytic model for the cool,T∼ 10⁴K, circumgalactic medium (CGM), describing the gas distribution, and thermal and ionization states. Our model assumes (total) pressure equilibrium with the ambient warm/hot CGM, photoionization by the metagalactic radiation, and allows for nonthermal pressure support, parameterized by the ratio of thermal pressures,η=P_hot,th/P_cool,th. We apply the model to the COS-Halos measurements and find that a nominal model withη= 3, gas distribution out tor≈ 0.6R_vir, andM_cool= 3 × 10⁹M_⊙, corresponding to a volume filling fraction off_V,cool≈ 1%, reproduces the Hiand low/intermediate metal ions (Cii, Ciii, Siii, Siiii, and Mgii) mean column densities. Variation of ±0.5 dex inηorM_coolencompasses ∼2/3 of the scatter between objects. Our nominal model underproduces the measured Civand Siivcolumns, and these can be reproduced with (i) a cool phase withM_cool∼ 10¹⁰M_⊙andη≈ 5, or (ii) cooling or mixing gas at intermediate temperatures, withM∼ 1.5 × 10¹⁰M_⊙and occupying ∼1/2 of the total CGM volume. For cool gas withf_V,cool≈ 1%, we estimate an upper limit on the cloud sizes,R_cl≲ 0.5 kpc. Our results suggest that for the average galaxy CGM, the mass and nonthermal support in the cool phase are lower than previously estimated, and extreme scenarios are not necessary. We estimate the rates of cool gas depletion and replenishment, and find accretion onto the galaxy can be offset, allowing ${\dot{M}}_{\mathrm{cool}}\approx 0$ over long timescales. 

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