R. Bindu

Importance of conduction electron correlation in a Kondo lattice, Ce2CoSi3

Kondo systems are usually described by the interaction of the correlation induced local moments with the highly itinerant conduction electrons. Here, we study the role of electron correlations among conduction electrons in the electronic structure of a Kondo lattice compound, Ce₂CoSi₃, using high resolution photoemission spectroscopy and ab initio band structure calculations, where Co 3d electrons contribute in the conduction band. High energy resolution employed in the measurements helped to reveal the signatures of Ce 4f states derived Kondo resonance features at the Fermi level and the dominance of Co 3d contributions at higher binding energies in the conduction band. The lineshape of the experimental Co 3d band is found to be significantly different from that obtained from the band structure calculations within the local density approximations, LDA. Consideration of electron-electron Coulomb repulsion, U, among Co 3d electrons within the LDA + U method leads to a better representation of experimental results. The signature of an electron correlation induced satellite feature is also observed in the Co 2p core level spectrum. These results clearly demonstrate the importance of the electron correlation among conduction electrons in deriving the microscopic description of such Kondo systems.

Local distortion of MnO6 octahedron in La1-xSrxMnO3+δ (X = 0.1-0.9) : an EXAFS study

Room-temperature Mn K-edge extended x-ray absorption fine structure (EXAFS) studies were carried out on La1−xSrxMnO3+δ (x = 0.1–0.9) compounds. It is found from the detailed EXAFS analysis that the local structure around Mn sites is different from the global structure inferred from x-ray diffraction, especially for x≤0.4, indicating the presence of local distortions in MnO6 octahedra. For the rhombohedral compounds, x = 0.1 to 0.3, the distortion is maximum for x = 0.1 and two bond lengths are seen: a short one in the basal plane and a long one in the apical plane. For compounds with x = 0.4–0.8 two short bonds in the basal plane and four long bonds (two in the basal plane and the remaining two in the apical plane) are seen. For the compounds with compositions up to x = 0.3, the long bond length decreases and the short bond length increases with increase in x, whereas for the compounds with 0.4≤x≤0.8 both types of bond length decrease. Such behaviour of bond lengths is an indication of the changed nature of distortion from Jahn–Teller type to breathing type at x = 0.4 composition.

Electronic and structural transition in La0.2Sr0.8MnO3

We investigate the interplay between the electronic and structural transitions in La0.2Sr0.8MnO3. The transport and specific heat measurements exhibit unusual evolution and the signature of a first order phase transition at around 265K. Mn K-edge extended x-ray absorption fine structure (EXAFS) results reveal distortion in the MnO6 octahedrons, even in the cubic phase, and a remarkable evolution of the distortion across the phase transition. These results manifest the importance of fluctuations in Mn 3d orbital occupancy on their electronic properties, which may help in understanding the orbital and spin ordering proposed in these systems.

Evidence of spin lattice coupling in MnTiO3: an x-ray diffraction study

Here we investigate the temperature evolution of the structural parameters of a potential magnetoelectric material, MnTiO3. The experimental results reveal interesting temperature dependence of the lattice parameters, which can be divided into three regions. In region I (300 K to 200 K), the lattice parameters show linear temperature dependence due to the thermal effect. In the region II (200 K to 95 K), lattice parameters show deviation from the linear behaviour due the competing intra-layer antiferromagnetic interactions setting in . The c/a ratio is seen to display a minima around 140 K. Below 140 K, the short Mn-O bonds increase suggesting the onset of inter-layer antiferromagnetic interaction . In region III (95 K to 23 K), the lattice parameter c shows negative thermal expansion where the antiferromagnetic interaction is fully established. The behaviour of the calculated Mn-O bonds based on first-principle calculations are in line with the experimental results. This study demonstrates the importance of spin lattice coupling in understanding the magnetic properties of the compound which is expected to be helpful in revealing the origin of magnetically induced ferroelectricity.

Complex evolution of the electronic structure of Cr with temperature

Employing state-of-the-art high resolution photoemission spectroscopy, we studied the electronic structure evolution of Cr with temperature. Experimental results reveal signature of a pseudogap much below the spin density wave transition temperature. A sharp peak appears near the Fermi level at low temperatures presumably related to the orbital Kondo effect. These results provide possible origin of the complex electronic properties observed in this system.

Spectral evolution in an insulator exhibiting linear specific heat

We investigate the spectral evolution of an antiferromagnetic insulator, La0.2Sr0.8MnO3, exhibiting linear specific heat, using state-of-the-art high resolution photoemission spectroscopy. Experimental spectral functions exhibit Fermi liquid-like energy dependence at all the temperatures studied. Room temperature spectra possess finite density of states at the Fermi level, which vanishes, generating a soft gap, at about 260 K (the magnetic transition temperature). High-resolution spectra reveal a hard gap in the magnetically ordered phase (C-type antiferromagnet). These results indicate an amorphous phase coexisting with the long-range ordered phase in these materials.

Complex temperature evolution of the electronic structure of CaFe2As2

Employing high resolution photoemission spectroscopy, we investigate the temperature evolution of the electronic structure of CaFe2As2, which is a parent compound of high temperature superconductors—CaFe2As2 exhibits superconductivity under pressure as well as doping of charge carriers. Photoemission results of CaFe2As2 in this study reveal a gradual shift of an energy band, α away from the chemical potential with decreasing temperature in addition to the spin density wave (SDW) transition induced Fermi surface reconstruction across SDW transition temperature. The corresponding hole pocket eventually disappears at lower temperatures, while the hole Fermi surface of the β band possessing finite p orbital character survives till the lowest temperature studied. These results, thus, reveal signature of complex charge redistribution among various energy bands as a function of temperature.

Structural evolution of MnTi0·8Ru0·2O3

Here We present the structural studies on MnTi1-xRuxO3(x=0, 0.2) compounds. The role of Ti ions in the magnetism of MnTiO3 was unclear so here this issue has been tried to address in this manuscript. The magnetic susceptibility data shows that the 3-dimensional magnetic character has been improved in the doped MnTiO3. The x=0 compound goes paramagnetic to antiferromagnetic phase at a temperature 64K followed by a broad peak at a temperature 100K . But in x=0.2 compound the antiferromagnetic transition temperature has been shifted towards the lower temperature to 47.5K with a broad peak at temperature 79K. On the doping of Ru at Ti site a sharp anomaly is observed 47.5K in the case of x=0.2 compound. This sharp anomaly attributes to the improved 3D character of magnetism in this compound which is weak in x=0 compound.

High Resolution Photoemission Study of Cr–A Classic SDW‐type Antiferromagnetic Metal

We studied the temperature evolution of the electronic structure of Chromium across the spin density wave‐type antiferromagnetic transition using high resolution photoemission spectroscopy. The spectral density of states (SDOS) remain almost unchanged across the magnetic transition. Signature of a pseudogap appears at low temperatures. In addition, SDOS exhibits growth of a peak just above the Fermi level at low temperatures similar to that found in various Kondo lattice systems.

Interesting spectral evolution in Fe-based superconductors

Fe-based superconductors are studied extensively during past decade to understand the interplay of superconductivity and magnetism. We studied the electronic structure of some of these fascinating systems exhibiting antiferromagnetic ordering and superconductivity, employing high resolution photoemission spectroscopy. We observed signature of finite hybridization of the electronic states corresponding to the local moment and the conduction electrons. The electronic states near Fermi level exhibit significant pnictogen/chalcogen p character. Signature of Kondo like features are observed near M-point in correlated Fe-compound.