| 1. |
- Brorsson, Joakim, 1988, et al.
(författare)
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Efficient Calculation of the Lattice Thermal Conductivity by Atomistic Simulations with Ab Initio Accuracy
- 2022
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Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 5:2
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Tidskriftsartikel (refereegranskat)abstract
- High-order force constant expansions can provide accurate representations of the potential energy surface relevant to vibrational motion. They can be efficiently parametrized using quantum mechanical calculations and subsequently sampled at a fraction of the cost of the underlying reference calculations. Here, force constant expansions are combined via the hiphive package with GPU-accelerated molecular dynamics simulations via the GPUMD package to obtain an accurate, transferable, and efficient approach for sampling the dynamical properties of materials. The performance of this methodology is demonstrated by applying it both to materials with very low thermal conductivity (Ba8Ga16Ge30, SnSe) and a material with a relatively high lattice thermal conductivity (monolayer-MoS2). These cases cover both situations with weak (monolayer-MoS2, SnSe) and strong (Ba8Ga16Ge30) pho renormalization. The simulations also enable to access complementary information such as the spectral thermal conductivity, which allows to discriminate the contribution by different phonon modes while accounting for scattering to all orders. The software packages described here are made available to the scientific community as free and open-source software in order to encourage the more widespread use of these techniques as well as their evolution through continuous and collaborative development.
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| 2. |
- Eriksson, Fredrik, 1992
(författare)
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Beyond Perturbation: Modeling Anharmonicity in Materials
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The vibrational motion of atoms is essential for understanding condensed matter systems. It directly influences numerous thermodynamic properties with significant technological implications. Additionally, these atomic movements indirectly contribute to a variety of electronic and optical properties, making the study of vibrations both challenging and rewarding. Today, we conceptualize these collective vibrational excitations as phonons. The study of phonons bridges theoretical and experimental approaches through computer simulations. Typically, phonons are studied at the lowest harmonic order and occasionally to the first order using perturbation theory. However, in many systems of interest, anharmonic motion, which represents interactions among phonons, is critical. Modeling this anharmonicity beyond perturbation theory is computationally intensive. Fortunately, recent advancements in various fields, both within and outside condensed matter physics, have made these simulations more feasible. In this thesis, the primary computational tools for studying phonons are outlined and applied to a diverse range of materials. Emphasis is placed on understanding the underlying dynamics through microscopic correlation functions and their link to experimental observables via spectral functions. Additionally, some practical details, often overlooked in the literature, are discussed. Specifically, the framework of lattice dynamics and the characterization of the potential energy surface through force constants are described. For complex systems that extend beyond perturbation theory, the complementary approach of molecular dynamics is explored, with a focus on phonon dynamics. These techniques are applied to two sets of materials currently of interest. First, anisotropic thermal conduction in rotationally disordered 2D van der Waals structures is examined using the Green-Kubo method. The findings align well with experimental results, demonstrating a substantial anisotropy that could be advantageous for managing thermal waste in integrated circuits. Moreover, the chemistry-independent suppression of through-plane thermal conductivity in these materials is shown. Additionally, through-plane thermal conduction as a function of the moiré twist angle is analyzed and correlated with an entropy measure. Lastly, the limitations of the quasi-particle picture of phonons are investigated in an inorganic halide perovskite. It is found that the soft phonons are overdamped over a wide temperature range above the phase transition but remain consistent with the model of independent damped harmonic oscillators.
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| 3. |
- Eriksson, Fredrik, 1992
(författare)
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Development and application of techniques for predicting and analysing phonon-derived materials properties
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thermodynamic properties of materials are of great interest for both scientists and engineers. A large contribution to many properties stems from the vibrational motion of the atoms in the material. An understanding of the dynamics of the vibrating atoms is therefore important for many other areas as well, including, e.g., electronic and optical properties. Since many materials of particular technological interest are crystalline, the vibrations can be studied in the framework of lattice dynamics. One of the main challenges in lattice dynamics is to acquire the force constants that describe the atomic interactions. Using crystal symmetries it is possible to reduce and cast this problem to a linear regression problem. This approach has been implemented in the present work in the hiphive package. The force constants (an interatomic potential) can be fitted to forces obtained from, e.g., density functional theory calculations. Although the problem of linear regression is well studied from a theoretical point of view the number of unknown coefficients in the force constant expansion is typically very large. Obtaining good models from limited data is possible via regularized regression, which has been successfully applied in many areas of physics. However, how well these techniques work in general for practical problems involving force constants is not well understood. By interfacing with the scikit-learn package, here, the hiphive package has been used to explore how well these techniques work in practice. It is found that many concepts from machine (or statistical) learning can be useful in order to predict macroscopic properties and quantify model uncertainties. Moving beyond the domain of pure lattice dynamics we also studied the thermal conductivity of rotationally disordered layered materials, which feature weak van-der-Waals interactions between the layers. These structures exhibit a remarkably low through-plane thermal conductivity and their dynamic properties can be described as one-dimensional glasses (a property worth further studies). By performing molecular dynamics simulations on state-of-the-art graphical processing units using the Green-Kubo formalism excellent agreement with experiments could be achieved.
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| 4. |
- Eriksson, Fredrik, 1992, et al.
(författare)
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The Hiphive Package for the Extraction of High-Order Force Constants by Machine Learning
- 2019
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Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 2:5
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Tidskriftsartikel (refereegranskat)abstract
- The efficient extraction of force constants (FCs) is crucial for the analysis of many thermodynamic materials properties. Approaches based on the systematic enumeration of finite differences scale poorly with system size and can rarely extend beyond third order when input data is obtained from first-principles calculations. Methods based on parameter fitting in the spirit of interatomic potentials, on the other hand, can extract FC parameters from semi-random configurations of high information density and advanced regularized regression methods can recover physical solutions from a limited amount of data. Here, the HIPHIVE Python package, that enables the construction of force constant models up to arbitrary order is presented. HIPHIVE exploits crystal symmetries to reduce the number of free parameters and then employs advanced machine learning algorithms to extract the force constants. Depending on the problem at hand, both over and underdetermined systems are handled efficiently. The FCs can be subsequently analyzed directly and or be used to carry out, for example, molecular dynamics simulations. The utility of this approach is demonstrated via several examples including ideal and defective monolayers of MoS2 as well as bulk nickel.
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| 5. |
- Eriksson, Fredrik, 1992, et al.
(författare)
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Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering
- 2023
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Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 17:24, s. 25565-25574
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Tidskriftsartikel (refereegranskat)abstract
- It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is, for example, of interest for applications in waste heat management in integrated circuits. Here, we show by atomic-scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials, including C and BN. In all three materials, introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to those of the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moiré structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4° depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.
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| 6. |
- Fransson, Erik, 1990, et al.
(författare)
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Efficient construction of linear models in materials modeling and applications to force constant expansions
- 2020
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Ingår i: npj Computational Materials. - : Springer Science and Business Media LLC. - 2057-3960. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Linear models, such as force constant (FC) and cluster expansions, play a key role in physics and materials science. While they can in principle be parametrized using regression and feature selection approaches, the convergence behavior of these techniques, in particular with respect to thermodynamic properties is not well understood. Here, we therefore analyze the efficacy and efficiency of several state-of-the-art regression and feature selection methods, in particular in the context of FC extraction and the prediction of different thermodynamic properties. Generic feature selection algorithms such as recursive feature elimination with ordinary least-squares (OLS), automatic relevance determination regression, and the adaptive least absolute shrinkage and selection operator can yield physically sound models for systems with a modest number of degrees of freedom. For large unit cells with low symmetry and/or high-order expansions they come, however, with a non-negligible computational cost that can be more than two orders of magnitude higher than that of OLS. In such cases, OLS with cutoff selection provides a viable route as demonstrated here for both second-order FCs in large low-symmetry unit cells and high-order FCs in low-symmetry systems. While regression techniques are thus very powerful, they require well-tuned protocols. Here, the present work establishes guidelines for the design of protocols that are readily usable, e.g., in high-throughput and materials discovery schemes. Since the underlying algorithms are not specific to FC construction, the general conclusions drawn here also have a bearing on the construction of other linear models in physics and materials science.
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| 7. |
- Fransson, Erik, 1990, et al.
(författare)
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Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites
- 2023
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Ingår i: Communications Physics. - 2399-3650. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- The soft modes associated with continuous-order phase transitions are associated with strong anharmonicity. This leads to the overdamped limit where the phonon quasi-particle picture can break down. However, this limit is commonly restricted to a narrow temperature range, making it difficult to observe its signature feature, namely the breakdown of the inverse relationship between the relaxation time and damping. Here we present a physically intuitive picture based on the relaxation times of the mode coordinate and its conjugate momentum, which at the instability approach infinity and the inverse damping factor, respectively. We demonstrate this behavior for the cubic-to-tetragonal phase transition of the inorganic halide perovskite CsPbBr3 via molecular dynamics simulations, and show that the overdamped region extends almost 200 K above the transition temperature. Further, we investigate how the dynamics of these soft phonon modes change when crossing the phase transition.
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| 8. |
- Ge, Yu, 1995, et al.
(författare)
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Experimental Validation of Single Base Station 5G mm Wave Positioning: Initial Findings
- 2022
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Ingår i: 2022 25th International Conference on Information Fusion, FUSION 2022. - Piscataway : IEEE.
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Konferensbidrag (refereegranskat)abstract
- 5G cellular networks can utilize millimeter wave signals, and support large bandwidths and large antenna arrays, which provide more geometric-based signals and higher delay and angle resolutions. These merits bring new opportunities in positioning the user with limited infrastructure through the use of combined angle and delay information. However, there are many practical challenges to overcome, in order to have a functioning single base station 5G mmWave positioning system. In this paper, we describe a deployed single base station mmWave positioning system, and provide an example of the measurement data. Furthermore, we perform measurement validation on a limited measurement data set by performing base station localization. Additional evaluations performed on simulation model data provide guidelines on the required size of the data set and receiver antenna configuration, which will be implemented in upcoming measurements.
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| 9. |
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| 10. |
- Holmer, Olov, 1992-, et al.
(författare)
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Optimal Aftertreatment Pre-Heat Strategy for Minimum Tailpipe NOx Around Green Zones
- 2020
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Ingår i: WCX SAE World Congress Experience. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- Green zones are challenging problems for the thermal management systems of hybrid vehicles. This is because within the green zone the engine is turned off, and the only way to keep the aftertreatment system warm is lost. This means that there is a risk of leaving the green zone with a cold and ineffective aftertreatment system, resulting in high emissions.A thermal management strategy that heats the aftertreatment system prior to turning off the engine, in an optimal way, to reduce the NOx emissions when the engine is restarted, is developed. The strategy is also used to evaluate under what conditions pre-heating is a suitable strategy, by evaluating the performance in simulations using a model of a heavy-duty diesel powertrain and scenario designed for this purpose.The results show that, for the studied vehicle, pre-heating of the aftertreatment system is an effective strategy to reduce NOx for engine-off events shorter than two hours, and is most effective for engine off events of around 1.5 hours. The results also show that for engine-off events longer than two hours, pre-heating quickly becomes an inefficient strategy. At this point, ammonia storage when the engine is turned off is more important, and pre-heating can even make the results worse, since an increased SCR temperature results in lower ammonia storage before turning off the engine, which is detrimental for NOx conversion during the restart.
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