Heavy fermions vs doped Mott physics in heterogeneous Ta-dichalcogenide bilayers.

Controlling and understanding electron correlations in quantum matter is one of the most challenging tasks in materials engineering. In the past years a plethora of new puzzling correlated states have been found by carefully stacking and twisting two-dimensional van der Waals materials of different kind. Unique to these stacked structures is the emergence of correlated phases not foreseeable from the single layers alone.

Ion-Selective Scattering Studied Using the Variable-Energy Electron Irradiation in the BaKFeAs Superconductor.

Low-temperature variable-energy electron irradiation was used to induce non-magnetic disorder in a single crystal of a hole-doped iron-based superconductor, Ba1-xKxFe2As2, = 0.80. To avoid systematic errors, the beam energy was adjusted non-consequently for five values between 1.

Assessing the source of error in the Thomas-Fermi-von Weizsäcker density functional.

We investigate the source of error in the Thomas-Fermi-von Weizsäcker (TFW) density functional relative to Kohn-Sham density functional theory (DFT). In particular, through numerical studies on a range of materials, for a variety of crystal structures subject to strain and atomic displacements, we find that while the ground state electron density in TFW orbital-free DFT is close to the Kohn-Sham density, the corresponding energy deviates significantly from the Kohn-Sham value. We show that these differences are a consequence of the poor representation of the linear response within the TFW approximation for the electronic kinetic energy, confirming conjectures in the literature.

The spontaneous symmetry breaking in TaNiSe is structural in nature.

The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order.

CoTe : A Quantum Critical Dirac Metal with Strong Spin Fluctuations.

Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity.

Thermodynamic stability of Li-B-C compounds from first principles.

Prediction of high- superconductivity in hole-doped LiBC two decades ago has brought about an extensive effort to synthesize new materials with honeycomb B-C layers, but the thermodynamic stability of Li-B-C compounds remains largely unexplored. In this study, we use density functional theory to characterize well-established and recently reported Li-B-C phases. Our calculation of the Li chemical potential in LiBC helps estimate the (,) conditions required for delithiation of the LiBC parent material, while examination of B-C phases helps rationalize the observation of metastable BC polymorphs with honeycomb and diamond-like morphologies.

Unconventional Pressure-Driven Metamagnetic Transitions in Topological van der Waals Magnets.

Activating metamagnetic transitions between ordered states in van der Waals magnets and devices bring great opportunities in spintronics. We show that external pressure, which enhances the interlayer hopping without introducing chemical disorders, triggers multiple metamagnetic transitions upon cooling in the topological van der Waals magnets Mn(BiSb)Te, where the antiferromagnetic interlayer superexchange coupling competes with the ferromagnetic interlayer coupling mediated by the antisite Mn spins. The temperature-pressure phase diagrams reveal that while the ordering temperature from the paramagnetic to ordered states is almost pressure-independent, the metamagnetic transitions show nontrivial pressure and temperature dependence, even re-entrance.

Prediction of unconventional magnetism in doped FeSb.

It is commonly believed that the energy bands of typical collinear antiferromagnets (AFs), which have zero net magnetization, are Kramers spin-degenerate. Kramers nondegeneracy is usually associated with a global time-reversal symmetry breaking (e.g.

Field-tunable toroidal moment in a chiral-lattice magnet.

Ferrotoroidal order, which represents a spontaneous arrangement of toroidal moments, has recently been found in a few linear magnetoelectric materials. However, tuning toroidal moments in these materials is challenging. Here, we report switching between ferritoroidal and ferrotoroidal phases by a small magnetic field, in a chiral triangular-lattice magnet BaCoSiO with tri-spin vortices.

Direct-Write of Nanoscale Domains with Tunable Metamagnetic Order in FeRh Thin Films.

We have directly written nanoscale patterns of magnetic ordering in FeRh films using focused helium-ion beam irradiation. By varying the dose, we pattern arrays with metamagnetic transition temperatures that range from the as-grown film temperature to below room temperature. We employ transmission electron microscopy, X-ray diffraction, and temperature-dependent transport measurements to characterize the as-grown film, and magneto-optic Kerr effect imaging to quantify the He irradiation-induced changes to the magnetic order.

Density functional theory-based electric field gradient database.

The deviation of the electron density around the nuclei from spherical symmetry determines the electric field gradient (EFG), which can be measured by various types of spectroscopy. Nuclear Quadrupole Resonance (NQR) is particularly sensitive to the EFG. The EFGs, and by implication NQR frequencies, vary dramatically across materials.

Double Indirect Interlayer Exciton in a MoSe/WSe van der Waals Heterostructure.

An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe/WSe heterostructure fabricated using state-of-the-art preparation techniques.

Superconductivity gets an iron boost.

Superconductivity, the resistance-free flow of electrical charges, is one of the most exotic phenomena in solid-state physics. Even though it was discovered almost a century ago, many questions remain unanswered, in particular those concerning the physics of high-temperature superconductivity. The recent discovery of iron-based superconductors was arguably the most important breakthrough in this field for more than two decades and may provide new avenues for understanding this high-temperature phenomenon.