Catherine Witteveen

We study the electronic structure of bulk 1T-TaSe_{2} in the charge-density wave phase at low temperature. Our spatially and angle-resolved photoemission data show insulating areas coexisting with metallic regions characterized by a chiral Fermi surface and moderately correlated quasiparticle bands. Additionally, high-resolution laser angle-resolved photoemission reveals variations in the metallic regions, with series of low-energy states, whose energy, number, and dispersion can be explained by the formation of quantum well states of different thicknesses. Dynamical mean field theory calculations show that the observed rich behavior can be rationalized by assuming occasional stacking faults of the charge density wave. Our results indicate that the diverse electronic phenomena reported previously in 1T-TaSe_{2} are dictated by the stacking arrangement and the resulting quantum size effects while correlation effects play a secondary role.

Gaining a deeper understanding of the interplay between charge density wave (CDW) order and superconductivity in transition metal dichalcogenides (TMDs), particularly within the (4H/2H)- Nb X 2 ( X = Se and S) family, remains an open and intriguing challenge. A systematic microscopic study across various compounds in this family is therefore required to unravel this complex interplay. Here, we report on muon spin rotation (µSR) and magnetotransport experiments investigating the effects of hydrostatic pressure on the superconducting transition temperature ( T c ), the temperature-dependent magnetic penetration depth ( λ eff ), and the charge density wave order (CDW) in two layered chalcogenide superconductors: 4H- NbSe 2 , which exhibits CDW order, and 2H- NbS 2 , which lacks such order. Our observations reveal a substantial 75 % enhancement of the superfluid density ( n s / m * ) in 4H- NbSe 2 upon the maximum applied pressure of ∼ 2 GPa, surpassing that of 2H- NbSe 2 . Despite the absence of CDW order, a sizable 20 % growth in superfluid density is also observed for 2H- NbS 2 under an applied pressure of 1.8 GPa. Notably, the evaluated superconducting gaps in all these TMDs remain largely unaffected by changes in applied pressure, irrespective of pressure-induced partial suppression of CDW order in (4H/2H)- NbSe 2 or its general absence in 2H- NbS 2 . These results underscore the complex nature of pressure-induced behaviors in these TMDs, challenging a simplistic view of competition solely between CDW order and superconductivity. Remarkably, the relationship between n s / m * and T c exhibits an unconventional correlation, indicating a noteworthy similarity with the behavior observed in cuprate, kagome, and iron-based superconductors.

Layered delafossite-type compounds and related transition metal dichalcogenides, characterized by their triangular net structures, serve as prototypical systems for exploring the intricate interplay between crystal structure and magnetic behavior. Herein, we report on the discovery of the compound K_1-xCrSe₂ (x ≈ 0.13), an incommensurately modulated phase. Single crystals of this compound were grown for the first time using a K/Se self-flux. We find a monoclinic crystal structure with incommensurate modulation that can be rationalized by a 3 + 1-dimensional model. This modulation compensates for the under-stoichiometry of K cations, creating pronounced undulations in the CrSe₂ layers. Our anisotropic magnetization measurements reveal that K_1-xCrSe₂ undergoes a transition to a long-range magnetically ordered state below T_N = 133 K, a temperature 1.6 to 3.3 times higher than in earlier reported KCrSe₂ compounds. Our findings open new avenues for tuning the magnetic properties of these layered materials through structural modulation.

We herein report on a fast and convenient soft-chemical synthesis approach towards large 1T-CrTe ₂ van-der-Waals crystals.

We report on the synthesis of LiCrTe 2 single crystals and on their anisotropic magnetic properties. We have obtained these single crystals by employing a Te/Li-flux synthesis method. We find LiCrTe 2 to crystallize in a TlCdS 2 -type structure with cell parameters of a = 3.9512(5) Å and c = 6.6196(7) Å at T = 175 K. The content of lithium in these crystals was determined to be neary stoichiometric by means of neutron diffraction. We find a pronounced magnetic transition at T N a b = 144 K and T N c = 148 K, respectively. These transition temperatures are substantially higher than earlier reports on polycrystalline samples. We have performed neutron powder diffraction measurements that reveal that the long-range low-temperature magnetic structure of single crystalline LiCrTe 2 is an A-type antiferromagnetic structure. Our DFT calculations are in good agreement with these experimental observations. We find the system to be easy axis with moments oriented along the c-direction experimentally as well as in our calculations. Thereby, the magnetic Hamiltonian can be written as H = H H e i s e n b e r g + ∑ i K c ( S i z ) 2 with K c = − 0.34 K (where | S z | = 3 2 ). We find LiCrTe 2 to be highly anisotropic, with a pronounced metamagnetic transition for H ⊥ a b with a critical field of μ H M M (5 K) ≈ 2.5 T. Using detailed orientation-dependent magnetization measurements, we have determined the magnetic phase diagram of this material. Our findings suggest that LiCrTe 2 is a promising material for exploring the interplay between crystal structure and magnetism, and could have potential applications in spin-based 2D devices.