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Item 3D MHD models of the centrifugal magnetosphere from a massive star with an oblique dipole field Get access Arrow(Monthly Notices of the Royal Astronomical Society, 2023-02-01) ud-Doula, Asif; Owocki, Stanley P.; Russell, Christopher; Gagné, Marc; Daley-Yates, SimonWe present results from new self-consistent 3D magnetohydrodynamics (MHD) simulations of the magnetospheres from massive stars with a dipole magnetic axis that has a non-zero obliquity angle (β) to the star’s rotation axis. As an initial direct application, we compare the global structure of co-rotating discs for nearly aligned (β = 5°) versus half-oblique (β = 45°) models, both with moderately rapid rotation (∼0.5 critical). We find that accumulation surfaces broadly resemble the forms predicted by the analytical rigidly rotating magnetosphere model, but the mass buildup to near the critical level for centrifugal breakout against magnetic confinement distorts the field from the imposed initial dipole. This leads to an associated warping of the accumulation surface towards the rotational equator, with the highest density concentrated in wings centred on the intersection between the magnetic and rotational equators. These MHD models can be used to synthesize rotational modulation of photometric absorption and H α emission for a direct comparison with observations.Item Ammonia dimer: extremely fluxional but still hydrogen bonded(Nature Communications, 2022-03-18) Aling, Jing; Szalewicz, Krzysztof; van der Avoird, AdIn the 1980s, Nelson, Fraser, and Klemperer (NFK) published an experimentally derived structure of the ammonia dimer dramatically different from the structure determined computationally, which led these authors to the question “Does ammonia hydrogen bond?". This question has not yet been answered satisfactorily. To answer it, we have developed an ab initio potential energy surface (PES) for this dimer at the limits of the current computational capabilities and performed essentially exact six-dimensional calculations of the vibration-rotation-tunneling (VRT) spectra of NH3-NH3 and ND3-ND3, obtaining an unprecedented agreement with experimental spectra. In agreement with other recent electronic structure calculations, the global minimum on the PES is in a substantially bent hydrogen-bonded configuration. Since the bottom of the PES is exceptionally flat, the dimer is extremely fluxional and the probability of finding it in configurations that are not hydrogen bonded is high. Nevertheless, the probability of hydrogen-bonded configurations is large enough to consider the ammonia dimer to be hydrogen bonded. We also show that NFK’s inference that the ammonia dimer is nearly rigid actually results from unusual cancellations between quantum effects that generate differences in spectra of different isotopologues.Item Analysis of neutron monitor count rates and timing distributions from latitude surveys(Journal of Physics: Conference Series, 2023-12-01) Yakum, P.; Khamphakdee, S.; Nuntiyakul, W.; Sáiz, A.; Ruffolo, D.; Evenson, P.; Bangliang, C.; Seripienlert, A.; Jiang, P.; Chuanraksasat, P.Neutron monitors continuously record the hadronic part of secondary atmospheric radiation on the ground, which originates from primary cosmic rays. In Thailand, we developed a mobile neutron monitor housed inside a standard-size shipping container named "Changvan." It contains three neutron-sensitive proportional counters set up in the typical NM64 layout. However, the central counter doesn't have the lead producer, leading us to refer to it as a "semi-leaded" neutron monitor. We examined cosmic ray spectral variations on two latitude surveys during 2018-2019 and 2019-2020. This work examines the ratio of count rates between leaded and unleaded setups, which shows notable variation based on geomagnetic cutoff rigidity, suggesting a sensitivity to the cosmic ray spectrum. This measurement could be implemented at stationary stations. The unleaded counter, shielded by the reflector with a higher count from nearby lead, may have advantages over a bare one. Furthermore, we explore alternative techniques to identify spectral changes in Galactic cosmic rays using Changvan data. We analyze using time delay histograms to determine the leader fraction (L) of neutrons that are not preceded by another neutron from the same primary cosmic ray. We also examine other parameters, including the alpha (α) parameter and pulse rate (PR), which can be compared with count rates (CR). Our findings indicate that the ratios of L and α are not significantly affected by geomagnetic cutoff rigidity. In contrast, CR and PR exhibit significant dependency and show opposite trends.Item An ‘analytic dynamical magnetosphere’ formalism for X-ray and optical emission from slowly rotating magnetic massive stars(Oxford University Press on behalf of the Royal Astronomical Society., 2016-08-01) Owocki, Stanley P.; ud-Doula, Asif; Sundqvist, Jon O.; Petit, Veronique; Cohen, David H.; Townsend, Richard H. D.; Stanley P. Owocki, Asif ud-Doula, Jon O. Sundqvist, Veronique Petit, David H. Cohen, and Richard H. D. Townsend; Owocki, Stanley P.Slowly rotating magnetic massive stars develop ‘dynamical magnetospheres’ (DMs), characterized by trapping of stellar wind outflow in closed magnetic loops, shock heating from collision of the upflow from opposite loop footpoints, and subsequent gravitational infall of radiatively cooled material. In 2D and 3D magnetohydrodynamic (MHD) simulations, the interplay among these three components is spatially complex and temporally variable, making it difficult to derive observational signatures and discern their overall scaling trends. Within a simplified, steady-state analysis based on overall conservation principles, we present here an ‘analytic dynamical magnetosphere’ (ADM) model that provides explicit formulae for density, temperature, and flow speed in each of these three components – wind outflow, hot post-shock gas, and cooled inflow – as a function of colatitude and radius within the closed (presumed dipole) field lines of the magnetosphere. We compare these scalings with time-averaged results from MHD simulations, and provide initial examples of application of this ADM model for deriving two key observational diagnostics, namely hydrogen H α emission line profiles from the cooled infall, and X-ray emission from the hot post-shock gas. We conclude with a discussion of key issues and advantages in applying this ADM formalism towards derivation of a broader set of observational diagnostics and scaling trends for massive stars with such dynamical magnetospheres.Item Application of collisional analysis to the differential velocity of solar wind ions(Frontiers in Astronomy and Space Sciences, 2024-01-09) Johnson, E.; Maruca, B. A.; McManus, M.; Stevens, M.; Klein, K. G.; Mostafavi, P.Collisional analysis combines the effects of collisional relaxation and large-scale expansion to quantify how solar wind parameters evolve as the plasma expands through the heliosphere. Though previous studies have applied collisional analysis to the temperature ratio between protons (ionized hydrogen) and α-particles (fully ionized helium), this is the first study to explore α-proton differential flow with collisional analysis. First, the mathematical model for the collisional analysis of differential flow was derived. Then, this model was applied to individual in-situ observations from Parker Solar Probe (PSP; r = 0.1–0.27 au) to generate predictions of the α-proton differential flow in the near-Earth solar wind. A comparison of these predicted values with contemporaneous measurements from the Wind spacecraft (r = 1.0 au) shows strong agreement, which may imply that the effects of expansion and Coulomb collisions have a large role in governing the evolution of differential flow through the inner heliosphere.Item Assessment of Directionally Solidified Eutectic Sm–Fe(Co)–Ti Alloys as Permanent Magnet Materials(IEEE Transactions on Magnetics, 2023-05-29) Gabay, Alexander M.; Han, Chaoya; Ni, Chaoying; Hadjipanayis, George C.Sm–Fe–Ti and Sm–Fe 0.8 Co 0.2 –Ti alloys were prepared via arc-melting and directionally solidified on a water-cooled copper hearth. The as-solidified alloys featured cells of the Sm(Fe,Co,Ti) 12 –Ti(Fe,Co) 2+δ –(α-Fe) lamellar eutectic. The lamellae of Sm(Fe,Co,Ti) 12 phase with a crystal structure of the ThMn12 type were less than 0.2 μm thick, and had their [001] easy-magnetization directions oriented along the temperature gradient of the solidification. The eutectic microstructure led to an increased coercivity, especially in the Co-added alloys. Below 250 °C, this coercivity was found not to vary much with temperature with a temperature coefficient of -0.18 %/°C. However, the modest absolute values, reaching only 0.7 kOe, are insufficient for utilization of the directionally solidified alloys as anisotropic permanent magnets.Item Band Engineering of ErAs:InGaAlBiAs Nanocomposite Materials for Terahertz Photoconductive Switches Pumped at 1550 nm(Advanced Functional Materials, 2024-04-18) Acuna, Wilder; Wu, Weipeng; Bork, James; Doty, Mathew F.; Jungfleisch, M. Benjamin; Gundlach, Lars; Zide, Joshua M. O.Terahertz technology has the potential to have a large impact in myriad fields, such as biomedical science, spectroscopy, and communications. Making these applications practical requires efficient, reliable, and low-cost devices. Photoconductive switches (PCS), devices capable of emitting and detecting terahertz pulses, are a technology that needs more efficiency when working at telecom wavelength excitation (1550 nm) to exploit the advantages this wavelength offers. ErAs:InGaAs is a semiconductor nanocomposite working at this energy; however, high dark resistivity is challenging due to a high electron concentration as the Fermi level lies in the conduction band. To increase dark resistivity, ErAs:InGaAlBiAs material is used as the active material in a PCS detecting Terahertz pulses. ErAs nanoparticles reduce the carrier lifetime to subpicosecond values required for short temporal resolution, while ErAs pins the effective Fermi level in the host material bandgap. Unlike InGaAs, InGaAlBiAs offers enough freedom for band engineering to have a material compatible with a 1550 nm pump and a Fermi level deep in the bandgap, meaning low carrier concentration and high dark resistivity. Band engineering is possible by incorporating aluminum to lift the conduction band edge to the Fermi level and bismuth to keep a bandgap compatible with 1550 nm excitation.Item Band structure and dispersion engineering of strongly coupled plasmon-phonon-polaritons in graphene-integrated structures(The Optical Society, 2016-01-19) Liu, Feng; Zhan, Tianrong; Zhu, Alexander Y.; Yi, Fei; Shi, Wangzhou; Feng Liu, Tianrong Zhan, Alexander Y. Zhu, Fei Yi, and Wangzhou Shi; Zhan, TianrongWe theoretically investigate the polaritonic band structure and dispersion properties of graphene using transfer matrix methods, with strongly coupled graphene plasmons (GPs) and molecular infrared vibrations as a representative example. Two common geometrical con- figurations are considered: graphene coupled subwavelength dielectric grating (GSWDG) and graphene nanoribbons (GNR). By exploiting the dispersion and the band structure, we show the possibility of tailoring desired polaritonic behavior in each of the two configurations. We compare the strength of coupling occurring in both structures and find that the interaction is stronger in GNR than that of GSWDG structure as a result of the stronger field confinement of the edge modes. The band structure and dispersion analysis not only sheds light on the physics of the hybridized polariton formation but also offers insight into tailoring the optical response of graphene light-matter interactions for numerous applications, such as biomolecular sensing and detection.Item Bimetal–organic frameworks derived tuneable Co nanoparticles embedded in porous nitrogen-doped carbon nanorods as high-performance electromagnetic wave absorption materials(Journal of Materials Chemistry C, 2021-05-04) Pan, Jiannan; Yang, Huadong; Hong, Qu; Wen, Hui-Min; Xiao, John Q.; Hu, JunThe in situ pyrolysis of metal–organic-frameworks (MOFs) is an effective strategy to prepare magnetic metal nanoparticle (NP) doped porous carbon materials. These composite materials have shown great potential as high performance electromagnetic wave (EMW) absorption materials. So far, the precise control of composite composition and structure has remained a major challenge in constructing highly porous composites with uniformly distributed NPs. In this work, we report a facile route to synthesize tuneable Co NPs embedded in porous nitrogen-doped carbon (Co/NC) nanorods through the direct thermolysis of the bimetal–organic framework (CoZn–ZIF) precursor. By adjusting the proportion of Co2+ in the MOF precursor, the content and distribution of Co NPs in the composite absorber change accordingly. When the molar ratio between Co2+ and Zn2+ is 3 : 1, the carbonized composites exhibit the largest external surface area and the best EMW absorption performance. With a filler mass loading of merely 15 wt%, the minimum reflection loss (RLmin) reaches −52.3 dB at 10.1 GHz with a thin layer thickness of 2.5 mm. The largest effective absorption bandwidth (EAB) of 5.0 GHz (11.1–16.1 GHz) is achieved in a 2.0 mm thick sample. The qualified bandwidth can be up to 14.5 GHz (3.5–18.0 GHz) with the integrated thickness from 1.0 mm to 5.5 mm. The enhanced conductive/magnetic losses, strong interfacial/dipolar/defect polarization, hierarchical pore structure and the geometric effect endow the Co/NC absorber with improved impedance matching and enhanced attenuation of EMW. This work provides a good direction for the future study of MOF-derived lightweight and efficient EMW absorbing materials.Item Bulk Mn-Al-C permanent magnets prepared by various techniques(AIP Publishing, 2016-03-01) Madugundo, Rajasekhar; Koylu-Alkan, Ozlem; Hadjipanayis, George C.; Rajasekhar Madugundo, Ozlem Koylu-Alkan, and George C. Hadjipanayis; Madugundo, Rajasekhar; Koylu-Alkan, Ozlem; Hadjipanayis, George C.Bulk Mn-Al-C magnets have been prepared by hot-compaction, microwave sintering and hot-deformation. Powders of Mn53.5Al44.5C2 alloy in the ε-phase produced by high energy ball milling have been used as precursor for the hot-compacted and microwave sintered magnets. Hot-deformed magnets were produced from alloy pieces in the τ-phase. The hot-compacted magnet exhibits magnetization, remanence and coercivity of 50 emu/g, 28 emu/g and 3.3 kOe, respectively. Microwave sintered magnet shows a maximum magnetization of 94 emu/g, remanence of 30 emu/g and coercivity of 1.1 kOe. The best magnetic properties are obtained in hot-deformed magnets with magnetization, remanence, coercivity and energy product of 82 emu/g, 50 emu/g, 2.2 kOe and 1.8 MGOe, respectively. Hot-deformed magnets exhibit texture with the highest degree of texture obtained 0.26. It is found that the pressure applied during compaction/deformation favors coercivity.Item Calculations of multipole transitions in Sn II for kilonova analysis(The European Physical Journal D, 2023-07-03) Bondarev, A. I.; Gillanders, J. H.; Cheung, C.; Safronova, M. S.; Fritzsche, S.We use the method that combines linearized coupled-cluster and configuration interaction approaches for calculating energy levels and multipole transition probabilities in singly ionized tin ions. We show that our calculated energies agree very well with the experimental data. We present probabilities of magnetic dipole and electric quadrupole transitions and use them for the analysis of the AT2017gfo kilonova emission spectra. This study demonstrates the importance and utility of accurate atomic data for forbidden transitions in the examination of future kilonova events.Item Cavity-Mediated Enhancement of the Energy Transfer in the Reduced Fenna–Matthews–Olson Complex(Journal of Chemical Theory and Computation, 2024-08-27) Herrera Rodríguez, Luis E.; Sindhu, Aarti; Rueda Espinosa, Kennet J.; Kananenka, Alexei A.Strong light-matter interaction leads to the formation of hybrid polariton states and can alter the light-harvesting properties of natural photosynthetic systems without modifying their chemical structure. In the present study, we computationally investigate the effect of the resonant cavity on the efficiency and the rate of the population transfer in a quantum system coupled to the cavity and the dissipative environment. The parameters of the model system were chosen to represent the Fenna–Matthews–Olson natural light-harvesting complex reduced to the three essential sites. The dynamics of the total system was propagated using the hierarchical equations of motion. Our results show that the strong light-matter interaction can accelerate the population transfer process compared to the cavity-free case but at the cost of lowering the transfer efficiency. The transition to the strong coupling regime was found to coincide with the degeneracy of polariton eigenvalues. Our findings indicate the potential and the limit of tuning the energy transfer in already efficient natural light-harvesting systems.Item Centrifugal breakout reconnection as the electron acceleration mechanism powering the radio magnetospheres of early-type stars(Monthly Notices of the Royal Astronomical Society, 2022-04-27) Owocki, S. P.; Shultz, M. E.; ud-Doula, A.; Chandra, P.; Das, B.; Leto, P.Magnetic B-stars often exhibit circularly polarized radio emission thought to arise from gyrosynchrotron emission by energetic electrons trapped in the circumstellar magnetosphere. Recent empirical analyses show that the onset and strength of the observed radio emission scale with both the magnetic field strength and the stellar rotation rate. This challenges the existing paradigm that the energetic electrons are accelerated in the current sheet between opposite-polarity field lines in the outer regions of magnetized stellar winds, which includes no role for stellar rotation. Building on recent success in explaining a similar rotation-field dependence of H α line emission in terms of a model in which magnetospheric density is regulated by centrifugal breakout (CBO), we examine here the potential role of the associated CBO-driven magnetic reconnection in accelerating the electrons that emit the observed gyrosynchrotron radio. We show in particular that the theoretical scalings for energy production by CBO reconnection match well the empirical trends for observed radio luminosity, with a suitably small, nearly constant conversion efficiency ϵ ≈ 10−8. We summarize the distinct advantages of our CBO scalings over previous associations with an electromotive force, and discuss the potential implications of CBO processes for X-rays and other observed characteristics of rotating magnetic B-stars with centrifugal magnetospheres.Item Combinational Vibration Modes in H2O/HDO/D2O Mixtures Detected Thanks to the Superior Sensitivity of Femtosecond Stimulated Raman Scattering(Journal of Physical Chemistry B, 2023-06-01) Pastorczak, Marcin; Duk, Katsiaryna; Shahab, Samaneh; Kananenka, Alexei A.Overtones and combinational modes frequently play essential roles in ultrafast vibrational energy relaxation in liquid water. However, these modes are very weak and often overlap with fundamental modes, particularly in isotopologues mixtures. We measured VV and HV Raman spectra of H2O and D2O mixtures with femtosecond stimulated Raman scattering (FSRS) and compared the results with calculated spectra. Specifically, we observed the mode at around 1850 cm–1 and assigned it to H–O–D bend + rocking libration. Second, we found that the H–O–D bend overtone band and the OD stretch + rocking libration combination band contribute to the band located between 2850 and 3050 cm–1. Furthermore, we assigned the broad band located between 4000 and 4200 cm–1 to be composed of combinational modes of high-frequency OH stretching modes with predominantly twisting and rocking librations. These results should help in a proper interpretation of Raman spectra of aqueous systems as well as in the identification of vibrational relaxation pathways in isotopically diluted water.Item Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations(Applied Physics Letters, 2023-02-13) Sharma, Vinay; Nepal, Rajeev; Wu, Weipeng; Pogue, E. A.; Kumar, Ravinder; Kolagani, Rajeswari; Gundlach, Lars; Jungfleisch, M. Benjamin; Budhani, Ramesh C.Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.Item Constraints on dark matter from dynamical heating of stars in ultrafaint dwarfs. II. Substructure and the primordial power spectrum(Physical Review D, 2024-10-15) Graham, Peter W.; Ramani, HarikrishnanThere is a large and growing interest in observations of small-scale structure in dark matter. We propose a new way to probe dark matter structures in the ∼10–108𝑀⊙ range. This allows us to constrain the primordial power spectrum over shorter distances scales than possible with direct observations from the CMB. For 𝑘 in the range ∼10–1000 Mpc−1 our constraints on the power spectrum are orders of magnitude stronger than previous bounds. We also set some of the strongest constraints on dark matter isocurvature perturbations. Our method relies on the heating effect such dark matter substructures would have on the distribution of stars in an ultrafaint dwarf galaxy. Many models of inflation produce enhanced power at these short distance scales and can thus be constrained by our observation. Further, many dark matter models such as axion dark matter, self-interacting dark matter and dissipative dark matter, produce dense structures which could be constrained this way.Item Cosmic-Ray Flux Correlation between MCMU and JBGO Neutron Monitors(The Astrophysical Journal, 2024-11-08) Kittiya, A.; Nuntiyakul, W.; Seripienlert, A.; Madlee, S.; Sonsrettee, W.; Evenson, P.; Ruffolo, D.; Sáiz, A.; Oh, S.; Jung, J.Neutron monitors (NMs) are large ground-based detectors of atmospheric secondary particles, mostly neutrons, from primary cosmic rays. Their sky direction and rigidity imply a well-defined incoming (asymptotic) direction in space. From 2015 December 16 to 2017 January 8, 6 of the 18 NM counters had been transferred from McMurdo to Jang Bogo, both in Antarctica, so data from similar detectors were recorded simultaneously at these two nearby NM stations. Autocorrelations of these NM count rates are well fit as the sum of three components: an exponential function and a cosine with a period of 1 day, both centered at zero lag, plus a constant. Fitting the cross correlation of the two count rates, the functions are no longer centered at zero lag. The best-fit cosine phase is at time lag −160.22 ± 0.12 minutes. Calculating cosmic-ray trajectories in Earth's magnetic field throughout the time interval, the mean difference in response-weighted asymptotic longitudes corresponds to time lag −169.41 ± 0.31 minutes, in close agreement with the observed lag. Thus, the cosine term is consistent with and provides a technique to cleanly measure the cosmic-ray anisotropy. In contrast, the peak term shows a time lag of –14.55 minutes, much closer to the –9.60 minutes lag in rotation due to the difference in geographic longitude. We find a similar behavior in the correlations between other pairs of stations. We propose that rapid fluctuations in the counting rate may be primarily due to cosmic-ray particles of very high energy.Item COVID-19 Questions for Physics Exams(The Physics Teacher, 2024-02-01) Wallace, Colin Scott; Deardorff, Duane; Young, Daniel; Churukian, Alice D.Physics instructors are always looking for questions and activities that both are tractable to students and illuminate real-life applications of physics. This is especially true in Introductory Physics for the Life Sciences (IPLS), where many students enter with the belief that physics is merely a “weed-out” course that has little to do with their majors and career goals.1 Authentic applications of physics to the life sciences help dispel this myth. The COVID-19 pandemic provides numerous examples of the relevance of physics to current events and the life sciences. Physics principles are used both to model the spread of the disease and to develop technologies that curb its transmission. Given the disruption COVID-19 has wrought on almost every aspect of life, physics content related to the pandemic inevitably captures students’ attention. Few topics are more relevant to their lives, at the time of writing, than COVID-19. While previous papers report on the effects of COVID-19 on physics instruction,2 COVID-related curricular materials have not been widely disseminated. This paper looks at how information about the disease can be incorporated into the assessment aspect of a class. Specifically, we share a sample of exam questions we have used to assess IPLS students since the beginning of the COVID-19 pandemic. Since they are exam questions, they are written such that they can be answered by a student in a short period of time as part of a larger timed assessment. Consequently, they are not meant to investigate every possible application of physics to COVID-19. They are also not meant to be collaborative activities that require a substantial amount of time, and potentially interdisciplinary knowledge to solve, although we have used them to assess the knowledge of students who have come through an IPLS sequence using such activities.3 Instructors looking for inspiration as they develop an activity may find these questions to be a fruitful launching point. These questions can also be used by other instructors on exams or developed into interactive in-class problem-solving activities.4 The questions below assess content from three different parts of our IPLS sequence: scaling, diffusion, and electricity. The following sections show the sample exam questions, describe their solutions and their relevance to our IPLS sequence, and discuss students’ performance in answering these questions.Item Criticality-Enhanced Magnetocaloric Effect in Quantum Spin Chain Material Copper Nitrate(Nature Publishing Group, 2017-03-15) Xiang, Jun-Sen; Chen, Cong; Li, Wei; Sheng, Xian-Lei; Su, Na; Cheng, Zhao-Hua; Chen, Qiang; Chen, Zi-Yu; Jun-Sen Xiang, Cong Chen, Wei Li, Xian-Lei Sheng, Na Su, Zhao-Hua Cheng, Qiang Chen & Zi-Yu Chen; Sheng, Xian-LeiIn this work, a systematic study of Cu(NO3)2·2.5 H2O (copper nitrate hemipentahydrate, CN), an alternating Heisenberg antiferromagnetic chain model material, is performed with multi-technique approach including thermal tensor network (TTN) simulations, first-principles calculations, as well as magnetization measurements. Employing a cutting-edge TTN method developed in the present work, we verify the couplings J = 5.13 K, α = 0.23(1) and Landé factors g∥= 2.31, g⊥ = 2.14 in CN, with which the magnetothermal properties have been fitted strikingly well. Based on first-principles calculations, we reveal explicitly the spin chain scenario in CN by displaying the calculated electron density distributions, from which the distinct superexchange paths are visualized. On top of that, we investigated the magnetocaloric effect (MCE) in CN by calculating its isentropes and magnetic Grüneisen parameter. Prominent quantum criticality-enhanced MCE was uncovered near both critical fields of intermediate strengths as 2.87 and 4.08 T, respectively. We propose that CN is potentially a very promising quantum critical coolant.Item Crystal Structure Predictions for 4-Amino-2,3,6-trinitrophenol Using a Tailor-Made First-Principles-Based Force Field(Crystal Growth and Design, 2022-01-24) Metz, Michael P.; Shahbaz, Muhammad; Song, Hongxing; Vogt-Maranto, Leslie; Tuckerman, Mark E.; Szalewicz, KrzysztofPredictions of crystal structures from first-principles electronic structure calculations and molecular simulations have been performed for an energetic molecule, 4-amino-2,3,6-trinitrophenol. This physics-based approach consists of a series of steps. First, a tailor-made two-body potential energy surface (PES) was constructed with recently developed software, autoPES, using symmetry-adapted perturbation theory based on a density-functional theory description of monomers [SAPT(DFT)]. The fitting procedure ensures asymptotic correctness of the PES by employing a rigorous asymptotic multipole expansion, which seamlessly integrates with SAPT(DFT) interaction energies. Next, crystal structure prediction (CSP) was performed by generating possible crystal structures with rigid molecules, minimizing these structures using the SAPT(DFT) force field, and running isothermal–isobaric molecular dynamics (MD) simulations with flexible molecules based on the tailor-made SAPT(DFT) intermolecular force field and a generic/SAPT(DFT) intramolecular one. This workflow led to the experimentally observed structure being identified as one of the forms with the lowest lattice energy, demonstrating the success of a first-principles, bottom-up approach to CSP. Importantly, we argue that the accuracy of the intermolecular potential, here the SAPT(DFT)-based potential, is determinative of the crystal structure, while generic/SAPT(DFT) force fields can be used to represent the intramolecular potential. This force field approach simplifies the CSP workflow, without significantly compromising the accuracy of the prediction.