Endless Dirac nodal lines and high mobility in kagome semimetal Ni3In2Se2: a theoretical and experimental study.

Kagome-lattice crystal is crucial in quantum materials research, exhibiting unique transport properties due to its rich band structure and the presence of nodal lines and rings. Here, we investigate the electronic transport properties and perform first-principles calculations for Ni3In2Se2kagome topological semimetal. First-principles calculations of the band structure without the inclusion of spin-orbit coupling (SOC) shows that three bands are crossing the Fermi level (EF), indicating the semi-metallic nature. With SOC, the band structure reveals a gap opening of the order of 10 meV.Z2index calculations suggest the topologically nontrivial natures (ν0;ν1ν2ν3) = (1;111) both without and with SOC. Our detailed calculations also indicate six endless Dirac nodal lines and two nodal rings with aπ-Berry phase in the absence of SOC. The temperature-dependent resistivity is dominated by two scattering mechanisms:s-dinterband scattering occurs below 50 K, while electron-phonon (e-p) scattering is observed above 50 K. The magnetoresistance (MR) curve aligns with the theory of extended Kohler's rule, suggesting multiple scattering origins and temperature-dependent carrier densities. A maximum MR of 120% at 2 K and 9 T, with a maximum estimated mobility of approximately 3000 cm2V-1s-1are observed. Ni3In2Se2is an electron-hole compensated topological semimetal, as we have carrier density of electron (ne) and hole (nh) arene≈nh, estimated from Hall effect data fitted to a two-band model. Consequently, there is an increase in the mobility of electrons and holes, leading to a higher carrier mobility and a comparatively higher MR. The quantum interference effect leading to the two dimensional (2D) weak antilocalization effect (-σxx∝ln(B)) manifests as the diffusion of nodal line fermions in the 2D poloidal plane and the associated encircling Berry flux of nodal-line fermions.

Structural and electronic transport properties of Zn- and Ga-doped Bi2-xSbxTe3-ySeytopological insulator single crystals.

A comprehensive study of structural and magnetotransport properties of pristine Bi2-xSbxTe3-ySey(BSTS) single crystals and doped with Zn (BSTS:Zn) and Ga (BSTS:Ga) are presented here. Magnetic field dependent Hall resistivities of the single crystals indicate that the holes are the majority carriers. The field dependent resistivity curves at different temperatures of the crystals display cusp-like characteristics at low magnetic fields, attributed to two-dimensional (2D) weak antilocalization (WAL) effect. We fit the observed low-field WAL effects at low temperatures using 2D and three-dimensional (3D) Hikami-Larkin-Nagaoka (HLN) equations. The 2D HLN equation fits the data more closely than the 3D HLN equation, indicating a 2D nature. The 2D HLN equation fit to the low field WAL effects at various temperatures reveal a phase coherence length (lφ) that decreases as temperature increases. The variation oflφwith temperature followsT-0.41power law for BSTS:Zn, suggesting that the dominant dephasing mechanism is a 2D electron-electron (e-e) interactions. For pristine BSTS and BSTS:Ga,lφ(T) is described by considering a coexistence of 2De-eand electron-phonon (e-p) interactions in the single crystals. The temperature variation of the longitudinal resistance in BSTS:Ga is described by 3D Mott variable range hoping model. In contrast, the transport mechanisms of both pristine BSTS and BSTS:Zn are described by a combination of 2D WAL/EEI models and 3D WAL.

An additional simultaneous magnetic ordering and magneto-capacitive behavior with dielectric relaxation besides multiferroicity in Fe1-xTexVO4.

Here we report the evidence of an additional magnetic ordering and frequency dispersive magneto-dielectric (MD) permittivity besides multiferroic behavior in Te4+(S= 0) doped FeVO4. Two antiferromagnetic transitions similar to FeVO4at ∼21.86 K (TN1) and 16.03 K (TN2) were observed in all samples. An additional novel defect clusters based magnetic ordering at relatively higher temperature (TAMO) ∼ 203 K is also observed from the magnetization. Evaluated magnetic moments show systematic decrease and the magnetic frustration factors show an increase with the increasing of Te4+(S= 0) content. MD studies show stable ferroelectric ordering at spiral magnetic transition (TN2) and the multiferroic order persists to the largest doping of Te (x= 0.10). The MD studies also reveal a magneto-capacitive (MC) behavior at TAMO(∼203 K) with a high dielectric constant and loss, and the possible reason for the magnetic ordering and MC behavior is ascribed to short range magnetic clustering arising out of defect based mechanisms. Mössbauer spectroscopic studies confirm local structural correlation with magnetic and ferroelectric ordering.

Vibrational spectra and optical properties of Fe1-xCrxVO4 solid solutions: With a group theory analysis.

The present manuscript reports vibrational spectra and optical studies of polycrystalline Fe1-xCrxVO4 solid solutions through FT-IR spectroscopy augmented with a group theory (G.T.) analysis and UV-Visible DRS spectroscopy. Full set of IR and Raman modes are determined by G. T. for various crystal symmetries in FeVO4-CrVO4 solid solutions where Triclinic, Monoclinic and Orthorhombic structures evolve with increasing Cr concentration. Experimentally obtained vibrational modes support the structural phase transitions and confirm formation of continuous solid solutions in Fe1-xCrxVO4. The Diffuse Reflectance Spectra (DRS) of Fe1-xCrxVO4 depicts the electronic structure and different optical transitions due to absorption of photon energy. The d-d transitions are manifested for all compounds in terms of crystal field stabilization energy (CFSE) caused by distorted lattice sites. The band gap energy of Fe1-xCrxVO4 is calculated using Tauc formula. It shows a red shift initially within triclinic structure then blue shift with the increase of Cr concentration. Urbach energy (Eu) tails in the spectra show the electronic structural disorder in Fe1-xCrxVO4 due to impurity energy levels of Cr ions within band gap region. It is observed that Eu decreases with the doping concentration due to the increase in crystal symmetry corresponding to the structural phase transitions in Fe1-xCrxVO4.

Vibrational spectra of Pb2Bi2Te3, PbBi2Te4, and PbBi4Te7 topological insulators: temperature-dependent Raman and theoretical insights from DFT simulations.

Herein, using Raman spectroscopy, we have presented the investigation of a temperature-dependent frequency shift and the line broadening of phonon modes by inserting the atomic layers of Pb and PbTe in the prototype 3D topological insulator Bi2Te3. Good quality single crystals of Pb2Bi2Te3, PbBi2Te4, and PbBi4Te7 were grown using the modified Bridgman technique. The Raman modes show progressive blue-shift with the decrease in temperature from 298 K to 93 K in Pb2Bi2Te3, PbBi2Te4, and PbBi4Te7 due to the anharmonic vibrations of the lattice as well as the increase in the strength of Bi-Te covalent interactions. The experimental results were complemented by extensive first principles calculations, where a reasonable matching between the experimental and computational data was found. Chemical pressure, induced by the insertion of Pb and PbTe layers in Bi2Te3, modified the interactions at the boundaries of the quintuple-layers, which was evident from the evolution of the A21u mode. The enhancement in the out-of-plane Bi-Te vibrations with respect to the in-plane Bi-Te vibrations was observed at low temperatures. The temperature coefficients of the Raman modes were useful in determining the thermal conductivity, which is a key design parameter for the fabrication of spintronic devices using topological insulators. The estimated first order temperature coefficient (χ') for Pb2Bi2Te3 signified the decrease in the thermal conductivity relative to Bi2Te3, which was caused by the insertion of the Pb layers in the van der Waals gaps of Bi2Te3.

Unusual Conductance Fluctuations and Quantum Oscillation in Mesoscopic Topological Insulator PbBi4Te7.

We present a detail study of Shubinikov-de-Haas (SdH) oscillations accompanied by conductance fluctuations in a mesoscopic topological insulator PbBi4Te7 device. From SdH oscillations, the evidence of Dirac fermions with π Berry phase is found and the experimentally determined two main Fermi wave vectors are correlated to two surface Dirac cones (buried one inside the other) of layered topological insulator PbBi4Te7. We have also found evidence of conductance fluctuations, the root mean square amplitude of which is much higher than the usual universal conductance fluctuations observed in nanometer size sample. Calculated autocorrelation functions indicate periodic unique fluctuations may be associated with the topological surface states in the compound.

Electronic, magnetic and spectroscopic properties of doped Mn(1-x) A x WO4 (A = Co, Cu, Ni and Fe) multiferroic: an experimental and DFT study.

The influence of dopants (Co, Cu, Fe and Ni) on the optical, electronic and magnetic properties of multiferroic MnWO4 was studied using Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), magnetization measurements and density functional theory (DFT) calculations. The evolution of Raman spectra with different elemental substitutions at the Mn site was also studied, where the peak width increased with doping of higher mass elements (Co, Cu, Fe and Ni). UV-Vis diffuse reflectance spectroscopy on polycrystalline Mn(1-x) A x WO4 (A = Co, Cu, Fe and Ni) (0 ⩽ [Formula: see text] ⩽ 0. was performed. The evaluated electronic band gap decreasing with successive Co, Cu and Fe doping reflected the lower ionic radius of the substituted element, and for Ni-doped MnWO4 the band gap increased slightly compared to the parent MnWO4. Bader charge transfer and a partial density of states (PDOS) analysis from DFT simulations predict the appearance of impurity states in the band gap region (of pure MnWO4) from the d orbital of the dopant (Co, Cu and Fe) hybridized with the p orbital of the bonded O atoms due to charge transfer from O to the dopant, and reduced the band gap of Co, Cu and Fe-doped MnWO4. On the other hand, for Ni-doped MnWO4 strong W-O hybridization occurring due to large charge transfer from oxygen to tungsten leads to an increase in the band gap. The band gap, computed using the GGA + U method, is close to the experimental value. The signature of the d-d transition observed in the UV spectra is explained in terms of the crystal field stabilization energy caused by the octahedral distortion present in the lattice. Three different antiferromagnetic phases (AF1, AF2 and AF3) are identified in MnWO4 and also for the Co (18.75%)-doped sample. For Cu-doped samples, suppression of the AF1 phase and stabilization of the AF2 phase is observed up to 2 K. Successive doping of Cu leads to the diminution of magnetic frustration. A new magnetic order is identified for Ni-doped MnWO4 in the temperature range 13.7-20 K.