Crystal growth, characterization and electronic band structure of TiSeS

Layered semimetallic van der Waals material $\mathrm{Ti}{\mathrm{Se}}_2$ has attracted a lot of attention because of interplay of a charge density wave (CDW) state and superconductivity. Its sister compound ${\mathrm{TiS}}_2$, being isovalent to $\mathrm{Ti}{\mathrm{Se}}_2$ and having the same crystal structure, shows a semiconducting behavior. The natural question rises—what happens at the transition point in $\mathrm{Ti}{\mathrm{Se}}_{2-x}{\mathrm{S}}_x$, which is expected for x close to one. Here we report the growth and characterization of TiSeS single crystals and the study of the electronic structure using density functional theory (DFT) and angle-resolved photoemission (ARPES). We show that TiSeS single crystals have the same morphology as $\mathrm{Ti}{\mathrm{Se}}_2$. Transport measurements reveal a metallic state; no evidence of CDW was found. DFT calculations suggest that the electronic band structure in TiSeS is similar to that of $\mathrm{Ti}{\mathrm{Se}}_2$, but the electron and hole pockets in TiSeS are much smaller. The ARPES results are in good agreement with the calculations.

Figure 1

(a) and (b) As-grown crystals of TiSeS by the chemical vapor transport, cell scale is 1 mm.

Figure 2

SEM image of an as-grown TiSeS crystal with topographical contrast (SE mode) in (a) and chemical contrast (BSE mode) in (b).

Figure 3

Powder x-ray diffraction pattern together with the Rietveld analysis of TiSeS.

Figure 4

As an example crystal structure of TiSeS (a) shown in the bc plane, (b) shown in the ac plane and (c) shown in the ab plane. The graphical representation was prepared using VESTA3 [33].

Figure 5

(a) Molar susceptibility as a function of temperature for a field of 1 T applied $H\parallel ab$ and $H\parallel c$ for TiSeS crystal. (b) Molar susceptibility as a function of temperature for a field of 1 T for TiSeS polycrystal. (c) Inverse molar susceptibility temperature dependence with Curie-Weiss linear fit.

Figure 6

Magneto-transport characterization of TiSeS single crystals. (a) Resistivity as a function of temperature for different crystal direction of the TiSeS single crystal. ${\rho }_{\text{xx}}$ and ${\rho }_{\text{yy}}$ were measured with electric current in the $ab$ plane and the ${\rho }_{\text{zz}}$ was measured with electric current applied along the $c$ direction. (b) Magneto resistance measured at longitudinal contacts at various temperatures when magnetic field is applied along the $c$ direction. (c) Hall measurements at various temperatures. (d) Linear slope was subtracted from the measured Hall signal to highlight the nonlinearity in magnetic field.

Figure 7

Lattice constants for ${\mathrm{TiX}}_2$ (X = S, Se) system, obtained with PBE in conjunction with zero dumping dispersion correction. Triangles stand for experimental data, circles are for calculated parameters.

Figure 8

Low energy Ti-Se-S structures, obtained with evolutionary search: (a) 156 symmetry, (b) 8 symmetry, (c) 1 symmetry, (d) 6 symmetry. Blue balls indicate Ti atoms, yellow and green are S and Se atoms, correspondingly.

Figure 9

ARPES spectra of TiSeS measured at 15 K along $\mathrm{\Gamma }$-M direction using (a) 70 eV and (d) 35 eV photon energy. (b) Energy distribution curve (EDC) taken at ${\mathrm{k}}_{\parallel }=0.5{\text{A}}^{-1}$. (c) Symmetrized Fermi surface map of TiSeS recorded with 80 eV photon energy. Brown dashed circle in center highlights electron pocket around $\mathrm{\Gamma }$ not clearly visible in the map due to strong intensity of electron pockets around M points. The electronlike pocket around $\mathrm{\Gamma }$ is clearly seen in panel (d) where we increase the dynamic range of the color scale enhancing the weak features. The dotted black hexagon represents the Brillouin zone. (e) Calculated band structure of experimental $\mathrm{Ti}{\mathrm{Se}}_2$. (f) Band structure spectral function in CPA approximation for TiSeS. (g) Fermi surface of $\mathrm{Ti}{\mathrm{Se}}_2$ with the structure parameters TiSeS; the color corresponds to the Fermi velocity