Robert Warmbier

Observation of anomalous magnetotransport properties in unexplored layered tetragonal compound SrMn ₂ Ge ₂ .

The structural, thermal, electrical transport, and magnetic properties of ternary compounds Ba T ₂ P ₂ ( T = Ru, Pd) are reported together with electronic structure calculations. Our results show that BaRu ₂ P ₂ crystallizes in the layered tetragonal ThCr ₂ Si ₂ -type structure (space group I 4/ mmm ) whereas BaPd ₂ P ₂ adopts the primitive tetragonal CeMg ₂ Si ₂ -type structure (space group P 4/ mmm ). The combined experimental results and electronic structure calculations suggest metallic and diamagnetic ground states for both materials. Electrical transport measurements further indicate non-Fermi liquid behavior at low temperatures. Notably, BaRu ₂ P ₂ exhibits an anomalous feature in the magnetic susceptibility at T ∼ 260 K, similar to that observed in BaRu ₂ As ₂ . A comparison between the bare density of states obtained from electronic structure calculations and the experimentally estimated values suggests the presence of strong correlation effects in the material. Furthermore, the heat capacity of BaPd ₂ P ₂ , calculated using the full phonon dispersion relation, is in good agreement with the experimental data, particularly at low temperatures, highlighting the significant contribution of optical phonon branches to C _p in this temperature range.

We report the growth of high-quality single crystals of hole-doped composition Sr 0.95 K 0.05 Co 2 As 2 and present an investigation of its structural, electronic, thermal, and magnetic properties. Our results show that hole doping leads to a substantial suppression of ferromagnetic (FM) correlations present in SrCo 2 As 2 leading to nearly isotropic magnetic susceptibilities χ a b , c ( T ) of Sr 0.95 K 0.05 Co 2 As 2 . The presence of FM fluctuations is considered one of the detrimental reasons why SrCo 2 As 2 does not achieve a superconducting ground state despite having a precursor for it in the form of stripe-like antiferromagnetic fluctuations. The suppression of FM fluctuations leads to qualitative similarities between the properties of hole-doped SrCo 2 As 2 and the end member of superconducting 122 iron-arsenide family KFe₂As₂. Our results also infer the presence of strongly correlated electronic states in Sr 0.95 K 0.05 Co 2 As 2 . Further, they show that the electronic structure of SrCo 2 As 2 as well as this hole-doped composition is exceptionally unstable in the vicinity of Fermi level E F for small alterations in the structural parameters and sensitively depends upon the details of simulation.

We report on the observation of a flat band situated at the Fermi level E F along with the structural, electrical transport, and magnetic properties of BaCo₂P₂ that crystallizes in the ThCr₂Si₂-type body-center tetragonal structure. This compound has the largest inter-layer pnictide (Pn) distance d Pn − Pn as well as the largest c / a ratio among all the known ACo 2 P n 2 (A = alkaline earth metal) compounds, where a and c are the tetragonal lattice parameters. Hence, the magnetic and electronic properties of this compound are expected to have a quasi-two-dimensional character. Despite the evidence of the presence of sizable magnetic interactions, magnetic susceptibility χ ( T ) of BaCo₂P₂ does not show magnetic ordering down to 1.8 K. The material shows good metallic conduction with a large residual resistivity ratio ρ 300 K / ρ 1.8 K ≈ 70 and a Fermi liquid behavior at low temperature. Kadowaki–Woods ratio R KW of BaCo₂P₂ suggests the presence of sizable electronic correlations within this system. Additionally, a large many-body enhancement of 2.3 of the experimental density of states D γ ( E F ) over the band-structure D band ( E F ) is inferred to arise from sizable electron-electron and/or electron-phonon interactions leading to a substantial deviation from the free-electron behavior.

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe–Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn–Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.

It is known that a 2D‐dimerized Su–Schrieffer–Heeger model can produce a nontrivial topological phase. It is a simple nearest‐neighbor model with four lattice sites in 2D. The Su–Schrieffer–Heeger model is easy to analyze but neglects important interaction in physical systems. Herein, an extended version of this model is proposed which includes all possible second‐nearest‐neighbor interactions in order to grant more flexibility when describing realistic systems. The addition of these interactions changes the symmetry of the model and as a result affects the topological properties. In order to characterize the topological changes to the model, a polarization invariant is used. It is further shown that these symmetry breaking interactions can be used to evoke a topological phase transition as well.

It is known that a two dimensional dimerized Su-Schrieffer-Heeger model can produce a nontrivial topological phase. It is a simple nearest-neighbor model with either two or four lattice sites in in two dimensions. Su-Schrieffer-Heeger model is easy to analyse but neglects important interaction in physical systems. In this work, an extended version of this model is proposed which includes all possible second nearest neighbor interactions in order to make it more feasible to describe realistic systems. The topological phases and properties of the model are characterized using a polarization invariant. It is further shown that second nearest neighbor interactions can be used to evoke a topological phase transition as well.

Classes of symmetry‐based model Hamiltonians can be constructed with specific point group restrictions. These models combine the flexibility of tight‐binding models with the analytical simplicity of the Su–Schrieffer–Heeger model and can be scaled in either direction. The applicability of models with two, three, and four bands to topological analysis is investigated and limitations to describing topological behaviour are assessed. The models are applied to fitting density functional theory (DFT) band structures for selected 2D materials with and without spin–orbit coupling. Suitable parameters for selected materials are obtained and used to describe the topological phase of the materials. Based on these results, a single two‐, three‐, and four‐band model is found which describes the band structure and topological properties of all the selected 2D materials. Suitable simplifications to the models are outlined to further illustrate how the analyzability can be scaled with the accuracy of the model. While generally a higher degree of symmetry helps the formation of topological phases, extremely symmetric point groups like restrict the allowed interactions to a degree that makes topological phases less easy to achieve.

Molybdenum disulfide (MoS₂), a two-dimensional (2D) material, promises better desalination efficiency, benefiting from the small diffusion length. While the monolayer nanoporous MoS₂ membrane has great potential in the reverse osmosis (RO) desalination membrane, multilayer MoS₂ membranes are more feasible to synthesize and economical than the monolayer MoS₂ membrane. Building on the monolayer MoS₂ membrane knowledge, the effects of the multilayer MoS₂ membrane in water desalination were explored, and the results showed that increasing the pore size from 3 to 6 Å resulted in higher permeability but with lower salt rejection. The salt rejection increases from 85% in a monolayer MoS₂ membrane to about 98% in a trilayer MoS₂ membrane. When averaged over all three types of membranes studied, the ions rejection follows the trend of trilayer > bilayer > monolayer. Besides, a narrow layer separation was found to play an important role in the successful rejection of salt ions in bilayer and trilayer membranes. This study aims to provide a collective understanding of this high permiselective MoS₂ membrane’s realization for water desalination, and the findings showed that the water permeability of the MoS₂ monolayer membrane was in the order of magnitude greater than that of the conventional RO membrane and the nanoporous MoS₂ membrane can have an important place in the purification of water.