Search

Electron transfer processes between a catalyst and a reactant molecule are inefficient beyond a couple of angstroms distance. However, the stabilizers of metal nanocrystals or ligands often create ...

Broad-band emissions related to self-trapped excitons (STEs) in the sub-bandgap region in organic–inorganic hybrid perovskites and double perovskite crystals have drawn attention in recent times due to their potential in optoelectronic device applications. In this study, we have shown that the formation of STEs in CH3NH3PbBr3 single crystals can be controlled using a suitable sample synthesis procedure. We have observed a broad-band emission (FWHM ∼ 80 nm) for crystals whose crystallization is fast, whereas it was absent if we follow slow crystallization procedures. Using UV-Visible absorption spectroscopy, temperature-dependent photoluminescence (PL), time-resolved PL (TRPL), and dc magnetization studies, we concluded that defect-assisted extrinsic self-trapping is dominant here over the intrinsic self-trapping process and excess Pb atoms in interstitial positions are predominantly responsible for the extrinsic self-trapping process.

We demonstrate antisolvent vapor assisted diffusion as a unique technique for controlled nucleation and better crystallization of methylammonium lead iodide, perovskite absorber film for solar cell applications. Our observations also suggest that due to non-covalent weak interaction between the perovskite absorber and the high boiling point antisolvent, Ethylacetate (EA), a new microstructured morphology (micro-rods) evolves in the film, which we report for the first time. It is believed that the changed morphology of the mesoporous films for the high boiling point antisolvent, exhibits better fill factor and results in 32% enhancement of power conversion efficiency than the low boiling point antisolvent, Dichloromethane (DCM), diffused and as-deposited films, thereby suggesting one more parameter to achieve better device performance.

Hybrid organic-inorganic lead halide perovskites are projected as new generation photovoltaic and optoelectronic materials with improved efficiencies. However, their electronic structure so far remains poorly understood, particularly in the orientationally disordered cubic phase. We performed electronic structure investigations using angle-resolved photoemission spectroscopy on two prototypical samples (MAPbBr3 and MAPbCl3) in their cubic phase, and the results are compared with the calculations within two theoretical models where MA+ is orientationally (1) disordered (MA+ ion is replaced by spherically symmetric Cs+ ion) and (2) ordered (MA oriented along (100) direction) but keeping the symmetry of the unit cell cubic. Degeneracy of the valence bands and behavior of constant energy contours are consistent with model 1, which supports strongly the disordered nature of the orientation of the MA+ ions in the cubic phase. Band structure calculations also reveal that spin-orbit coupling induced Rashba splitting is suppressed by the orientational disorder.

A spark-plasma sintered ${\mathrm{GdCrO}}_{3}$ (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The intrinsic nature of this room-temperature ferroelectricity is established using ferroelectric positive-up--negative-down measurements and supported through piezoresponce force microscopy measurements. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. This is contrast to the concomitant antiferromagnetic and ferroelectric ordering observed below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in ${\mathrm{GdCrO}}_{3}$ (GCO) (prepared using standard solid-state synthesis technique). Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic $Pna{2}_{1}$ space group (the reported low-temperature ferroelectric phase in GCO), while GCO stabilizes in the centrosymmetric $Pbnm$ space group at room temperature. Using first-principles calculations, we investigated the relative energies among various possible structures of ${\mathrm{GdCrO}}_{3}$ and found that the orthorhombic $Pna{2}_{1}$ and $Pbnm$ space groups are the most stable structures. The ferroelectric $Pna{2}_{1}$ phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond $\ensuremath{\approx}450\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric $Pbnm$ phase.

Solar cells based on new class of organic inorganic hybrid perovskite CH3NH3PbI3 have sprung to prominence in recent years, due to their remarkable improvements of photovoltaic power conversion efficiencies (PCEs) over the last few years. Recently, it was observed that antisolvent engineering was an effective approach to improve the PCE and stability of perovskite solar cells. In this direction CH3NH3PbI3 based thin film solar cells were prepared by Ethyl acetate (EA); antisolvent treatment for the first time. Our observations suggest that due to non-covalent weak interaction between the perovskite absorber and the high boiling point antisolvent, EA, a new micro structured morphology evolves in the CH3NH3PbI3 thin film, which we report for the first time. FESEM image shows microrod type structures of CH3NH3PbI3 after EA antisolvent treatment. Energy band diagram was constructed using photoluminescence and photoemission studies. A better power conversion efficiency was achieved in EA treated film compare to without EA treated film.

We report a new route to synthesize high-quality, large-size crystals of CH3NH3PbCl3 through proper selection of DSMO–GBL solution via solvent evaporation method at room temperature. A detailed evaluation of the structural, electronic, optical and electrical properties of these crystals was carried out. Our XPS studies suggested that organic–inorganic halide perovskites are sensitive to X-ray-induced damage, and hence, their properties may get altered. Also, photoluminescence studies displayed two peak spectra, indicating coexistence of order–disorder domains of CH3NH3 in the sample. Further observation of low defect concentration and longer diffusion length indicates that crystals grown by the presented method can offer promising solutions for optoelectronic devices.

The question on whether there exists a unique photoelectron reference plane for a stepped solid surface is discussed on the basis of angle-resolved photoelectron spectroscopy data for Ag films grown on Pt(997). Different step morphologies at the surface and interface, revealed by low-energy electron diffraction measurements, result in distinctly different band dispersions of the sp-like quantum well states and of the Shockley surface state. Quantum well standing waves form between the parallel optical surface and interface planes, while the surface state follows the orientation of a local plane tilted with respect to the optical surface. These findings show the connection of the photoelectron reference plane with the local morphology of a solid surface and the spatial extent of the electron wave functions., This work has been partially funded through the project EUROFEL-ROADMAP ESFRI and by the Spanish Ministry of Science and Innovation (Grant No. MAT-2017-88374-P) and the Basque Government (Grant No. IT-1255-19)

Physical review / B 101(6), 064409 (2020). doi:10.1103/PhysRevB.101.064409, Simultaneous coexistence of room-temperature ferromagnetism and ferroelectricity in Fe-doped BaTiO$_3$ (BTO) is intriguing, as such Fe doping into tetragonal BTO, a room-temperature ferroelectric, results in the stabilization of its hexagonal polymorph which is ferroelectric only below ∼80K. Here, we investigate its origin and show that Fe-doped BTO has a mixed-phase room-temperature multiferroicity, where the ferromagnetism comes from the majority hexagonal phase and a minority tetragonal phase gives rise to the observed weak ferroelectricity. In order to achieve majority tetragonal phase (responsible for room-temperature ferroelectricity) in Fe-doped BTO, we investigate the role of different parameters which primarily control the paraelectric hexagonal phase stability over the ferroelectric tetragonal one and identify three major factors, namely the effect of ionic size, Jahn-Teller (JT) distortions, and oxygen-vacancies, to be primarily responsible. The effect of ionic size which can be qualitatively represented using the Goldschmidt's tolerance factor seems to be the major dictating factor for the hexagonal phase stability. The understanding of these factors not only enables us to control them but also to achieve a suitable codoped BTO compound with enhanced room-temperature multiferroic properties., Published by Inst., Woodbury, NY

Even though tandem solar cells comprised of mixed-halide perovskites CH3NH3Pb(I1–xBrx)3 were expected to have much higher efficiency, the observation that they undergo photoinduced phase separation/demixing put forth a limitation to their possible utility. Herein, using temperature dependent photoluminescence studies, we show that the stated photoinduced phase separation occurs only in a narrow temperature range and above a particular bromine concentration. Our observation of the disappearance of phase separation at elevated temperatures suggests the possibility that these tandem solar cells may indeed work better at elevated temperatures. Further, we provide the first experimental proof for the demixing transition temperature as predicted by Bischak et al. and also observe that demixing and remixing temperatures are pinned to crystallographic phase transition temperatures. Longer carrier lifetime of iodide-rich clusters is observed confirming the strong electron–phonon interaction (polaronic effect) whic...

Journal of alloys and compounds 883, 160862 (2021). doi:10.1016/j.jallcom.2021.160862, We report structural, electronic, magnetic, and magnetocaloric studies of polycrystalline DyMnO$_3$ and its A-site Gd substituents, Dy$_{1−x}$Gd$_x$MnO$_3$, where 0 ≤ x ≤ 1. Detailed comparison of various magnetocaloric parameters shows that Dy$_{0.5}$Gd$_{0.5}$MnO$_3$ has the best values with a large isothermal magnetic entropy change, -$\Delta S^{max}_m$ ~ 15.35 J/kg K, adiabatic temperature change, $\Delta T^{max}_{ad}$ ~ 6.15 K and relative cooling power, RCP ~ 307 J/kg, suggesting that rare earth mixing plays a vital role in enhancing the magnetocaloric properties. In addition, absence of magnetic and thermal hysteresis also makes it one of the best candidates for magnetic refrigeration among all known potential candidates for cryogenic temperature applications., Published by Elsevier, Lausanne

Cobalt and magnesium sulfates of the compositions, [C4N2H12]2[Co3F2(SO4)3(H2O)2], (1) and [NH4]2[Mg3(OH)2(SO4)3(H2O)2], (2), respectively, have been synthesized under hydro/solvothermal conditions, and are characterized by IR spectra, elemental analysis, powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and X-ray single-crystal diffraction. 1 and 2 crystallized in the orthorhombic space groups Pnma and Cmc2 respectively. While 1 is templated by the organic piperazinium cation, 2 is obtained in the presence of ammonium ions. The layered structures are formed by the diamond chains comprising of M3(μ3-OH/F)2 units (M = Co: 1, Mg: 2). Magnetic studies of 1 reveal its ferromagnetic nature with a transition at 10.8 K and show it does not exhibit spin-glass freezing. Isothermal magnetization shows a hysteresis loop at 2.5 K with a coercive field of 1200 Oe and remnant magnetization of 0.1μB. A sharp λ-like anomaly is also seen in the heat capacity curve, favoring long range magnetic ordering below Tc.

Hybrid organic–inorganic metal halides of the type CH3NH3PbX3 have emerged as potential materials for photovoltaic applications. In this paper we discuss structural, electronic, and optical spectroscopy investigations performed on high quality single crystals of CH3NH3PbI3. Our results conclusively suggest that CH3NH3PbI3 crystallizes in centrosymmetric space group and the methylammonium moiety exhibits disordered packing at room temperature. Extracted values of the exciton binding energy, the electron–phonon coupling constant, and the schematic energy level diagram constructed from the emission broadening, Raman, and photoemission spectroscopy measurements clearly show the potential of this system in photovoltaic applications.

We report on detailed structural, electronic, and magnetic studies of ${\mathrm{GdMn}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ for Cr doping levels; $x=0 \ensuremath{\le}\phantom{\rule{4pt}{0ex}}x\phantom{\rule{4pt}{0ex}}\ensuremath{\le}$ 1. X-ray diffraction studies suggest that ${\mathrm{GdMn}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_{3}$ has a monoclinic $P{2}_{1}/b$ structure with alternate arrangements of Mn and Cr atoms along the [001] direction. In the solid solutions, the Jahn-Teller distortion associated with ${\mathrm{Mn}}^{3+}$ ions gives rise to major changes in the $bc$-plane sublattice and also an effective orbital ordering in the $ab$ plane, which persist up to compositions $x\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}$ 0.35. These distinct features in the lattice and orbital degrees of freedom are also correlated with $bc$-plane anisotropy of the local Gd environment. A gradual evolution of electronic states with doping is also clearly seen in O $K$-edge x-ray absorption spectra. Evidence of magnetization reversal in field-cooled-cooling mode for $x\phantom{\rule{4pt}{0ex}}\ensuremath{\ge}$ 0.35 coinciding with the Jahn-Teller crossover suggests a close correlation between magnetic interaction and structural distortion. These observations indicate a strong entanglement between lattice, spin, electronic, and orbital degrees of freedom. The nonmonotonic variation of remnant magnetization can be explained by doping-induced modification of magnetic interactions. Density-functional-theory calculations are consistent with layer-by-layer-type arrangements of Cr ions and Mn ions with ferromagnetic (antiferomagnetic) coupling between Mn (Cr) ions for intermediate compounds ($x=0.5$). For $x=0.25$ compositions, we found alternate layers of Mn and mixed Mn-Cr atoms stacked along the $c$ axis with intralayer ferromagnetic coupling and interlayer antiferromagnetic coupling. For $x=0.75$ compositions, there exists strong antiferomagnetic coupling between half-filled ${t}_{2g}$ orbitals of in-plane Cr ions along with a feromagnetic Mn-Cr coupling.

Physical review / B covering condensed matter and materials physics 100(11), 115162 (2019). doi:10.1103/PhysRevB.100.115162, The presence of orbital degree of freedom in strongly correlated systems leads to unusual orderings. In this paper we use a combination of density-functional theory calculations and various experimental investigations to reveal a unique ground state for a Ti 3 + containing spinel oxide, MnTi2O4, which hosts an exotic combination of a rare tetramer orbital (associated with Ti3+ 3d1 electron) ordering along equivalent ⟨111⟩ directions involving all three t2g orbitals, a ferrimagnetic Mn-Ti, and a ferromagnetic Ti-lattice spin ordering. A combination of spin-orbital superexchange and Jahn-Teller-effect related strain-energy optimization provides a microscopic understanding for the stabilization of the unique ground state, which is found to be also electrically polar. The tetramer orbital ordering induces Ti-Ti bond length modulations and the short and long Ti-Ti bonds form helices around the crystallographic c-axis with a particular winding direction, causing the ground state structure to become spatial chiral., Published by Inst., Woodbury, NY

We have studied the structural phase transformation in thin Co films using moderate energy Au ion irradiation. We found that local energy transfer along the track of the ion beam is an effective method for transforming fcc phase of Co into technologically appealing hcp phase. As a function of the irradiation fluence, a gradual crossover to the hcp phase was observed in X-ray diffraction data. Such phase transition was explained on the basis of evolution of grains under increasing ion fluence. Bloch-Gruneisen simulations to the four contact resistivity data showed a decrease in Debye temperature with evolution of hcp-phase Cobalt, consistent with the reduced modal freedom for phonons in the hcp structure compared to the fcc structure.

Applied physics letters 117(1), 012901 (2020). doi:10.1063/5.0004785, Fe doping into BaTiO$_3$ stabilizes the paraelectric hexagonal phase in place of the ferroelectric tetragonal one. We show that simultaneous doping of Bi along with Fe into BaTiO$_3$ effectively enhances the magnetoelectric (ME) multiferroic response (both ferromagnetism and ferroelectricity) at room temperature, through careful tuning of Fe valency along with the controlled recovery of the ferroelectric-tetragonal phase. We also report a systematic increase in large dielectric constant values as well as reduction in loss tangent values with relatively moderate temperature variation of the dielectric constant around room temperature with increasing Bi doping content in Ba$_{1���x}$BixTi$_{0.90}$Fe$_{0.10}$O$_3$ (0 ��� x ��� 0.10), which makes the higher Bi���Fe codoped sample (x = 0.08) promising for use as a room-temperature high-�� dielectric material. Interestingly, the x = 0.08 (Bi���Fe codoped) sample is not only found to be ferroelectrically (���20 times) and ferromagnetically (���6 times) stronger than x = 0 (only Fe-doped) at room temperature, but also observed to be better insulating (larger bandgap) with indirect signatures of larger ME coupling as indicated from anomalous reduction of the magnetic coercive field with decreasing temperature. Thus, room-temperature ME multiferroicity has been engineered in Bi and Fe codoped BTO (BaTiO$_3$) compounds., Published by American Inst. of Physics, Melville, NY

Physical review / B 97(22), 224107 (2018). doi:10.1103/PhysRevB.97.224107, emperature-dependent synchrotron x-ray diffraction and extended x-ray absorption fine structure (EXAFS) studies were performed to understand the role of structural characteristics in driving the magnetoelectric multiferroic properties of GdCrO$_{3}$. The results suggest that the distortion in the structure appears to be associated with the off-center displacement of Gd atoms together with octahedral rotations via displacement of the oxygen ions in GdCrO$_{3}$. Further, a comparative EXAFS study of GdCrO$_{3}$ with a similar system, YCrO$_{3}$, suggests that the oxygen environment of Gd in GdCrO$_{3}$ is different from that of Y in YCrO$_{3}$, which results in an orthorhombic Pna2$_{1}$ structure in GdCrO$_{3}$ in contrast to the monoclinic P2$_{1}$ structure in YCrO$_{3}$., Published by APS, Woodbury, NY

Nanoscale 10(45), 21396 - 21405 (2018). doi:10.1039/C8NR06844E, Despite extensive use of Pd nanocrystals as catalysts, the realization of a Pd-based continuous flow reactor remains a challenge. Difficulties arise due to ill-defined anchoring of the nanocrystals on a substrate and reactivity of the substrate under different reaction conditions. We demonstrate the first metal (Pd) nanowire-based catalytic flow reactor that can be used across different filtration platforms, wherein, reactants flow through a porous network of nanowires (10–1000 nm pore sizes) and the product can be collected as filtrate. Controlling the growth parameters and obtaining high aspect ratio of the nanowires (diameter = ∼13 nm and length > 8000 nm) is necessary for successful fabrication of this flow reactor. The reactor performance is similar to a conventional reactor, but without requiring energy-expensive mechanical stirring. Synchrotron-based EXAFS studies were used to examine the catalyst microstructure and Operando FT-IR spectroscopic studies were used to devise a regenerative strategy. We show that after prolonged use, the catalyst performance can be regenerated up to 99% by a simple wash-off process without disturbing the catalyst bed. Thus, collection, regeneration and redispersion processes of the catalyst in conventional industrial reactors can be avoided. Another important advantage is avoiding specific catalyst-anchoring substrates, which are not only expensive, but also non-universal in nature., Published by RSC Publ., Cambridge

The confinement effects of electrons in ultrathin films and nanowires grown on metallic and semiconducting substrates investigated using band mapping of their electronic structures using angle-resolved photoemission spectroscopy is discussed here. It has been shown that finite electron reflectivity at the interface is sufficient to sustain the formation of quantum well states and weak quantum well resonance states even in closely matched metals. The expected parabolic dispersion of sp-derived quantum well states for free-standing layers undergoes deviations from parabolic behaviour and modifications due to the underlying substrate bands, suggesting the effects of strong hybridization between the quantum well states and the substrate bands. Electron confinement effects in low dimensions as observed from the dispersionless features in the band structures are also discussed.

The study reports the observation of radial vibrational modes in ultra-thin walled anatase TiO2 nanotube powders grown by rapid breakdown anodization technique using resonant Raman spectroscopic study. The as-grown tubes in the anatase phase are around 2–5 nm in wall thickness, 15–18 nm in diameter and few microns in length. The Eg(ν1,ν5,ν6) phonon modes with molecular vibrations in the radial direction are predominant in the resonance Raman spectroscopy using 325 nm He–Cd excitation. Multi-phonons including overtones and combinational modes of Eg(ν1,ν5,ν6) are abundantly observed. Frohlich interaction owing to electron–phonon coupling in the resonance Raman spectroscopy of ultra-thin wall nanotubes is responsible for the observation of radial vibrational modes. Finite size with large surface energy in these nanotubes energetically favor only one mode, B1g(ν4) with unidirectional molecular vibrations in the parallel configuration out of the three Raman modes with molecular vibration normal to the radial modes. Enhanced specific heat with increasing temperatures in these nanotubes as compared to that reported for nanoparticles of similar diameter may possibly be related to the presence of the prominent radial mode along with other energetic phonon mode. The findings elucidate the understanding of total energy landscape for TiO2 nanotubes. Copyright © 2015 John Wiley & Sons, Ltd.

Our detailed temperature dependent synchrotron powder x-ray diffraction studies along with first-principles density functional perturbation theory calculations, enable us to shed light on the origin of ferroelectricity in GdCrO3. The actual lattice symmetry is found to be noncentrosymmetric orthorhombic Pna21 structure, sup- porting polar nature of the system. Polar distortion is driven by local symmetry breaking and by local distortions dominated by Gd off-centering. Our study reveals an intimate analogy between GdCrO3 and YCrO3. However, a distinctive difference exists that Gd is less displacive compared to Y, which results in an orthorhombic P na21 structure in GdCrO3 in contrast to monoclinic structure in YCrO3 and consequently, decreases its polar property. This is due to the subtle forces involving Gd-4f electrons either directly or indirectly. A strong magneto-electric coupling is revealed using Raman measurements based analysis in the system below Cr-ordering temperature, indicating their relevance to ferroelectric modulation., Comment: 8 pages, 6 figures

We present our angle resolved photoelectron spectroscopy (ARPES) and density functional theory results on quaternary topological insulator (TI) BiSbTe1.25Se1.75 (BSTS) confirming the non-trivial topology of the surface state bands (SSBs) in this compound. We find that the SSBs, which are are sensitive to the atomic composition of the terminating surface have a partial 3D character. Our detailed study of the band bending (BB) effects shows that in BSTS the Dirac point (DP) shifts by more than two times compared to that in Bi2Se3 to reach the saturation. The stronger BB in BSTS could be due to the difference in screening of the surface charges. From momentum density curves (MDCs) of the ARPES data we obtained an energy dispersion relation showing the warping strength of the Fermi surface in BSTS to be intermediate between those found in Bi2Se3 and Bi2Te3 and also to be tunable by controlling the ratio of chalcogen/pnictogen atoms. Our experiments also reveal that the nature of the BB effects are highly sensitive to the exposure of the fresh surface to various gas species. These findings have important implications in the tuning of DP in TIs for technological applications.

Cobalt and magnesium sulfates of the compositions, [C

Solar cells based on new class of organic inorganic hybrid perovskite CH3NH3PbI3 were prepared by Ethyl acetate (EA); antisolvent treatment for the first time. This treatment results in new morphology for CH3NH3PbI3 thin film. FESEM image shows microrod type structures of CH3NH3PbI3 after EA antisolvent treatment. Energy band diagram was constructed using photoluminescence and photoemission studies. A better power conversion efficiency was achieved in EA treated film compare to without EA treated film.

We report the experimental study of low temperature thermoelectric properties of ternary half-Heuslar LuPdBi, synthesized by conventional arc melting. Resistivity measurements reveal the semi metallic nature of the conduction behavior, where holes are the major charge carriers as confirmed from positive value Seebeck coefficient in the entire temperature range 18–350 K. Thermal conductivity behavior indicates the dominance of phonon contribution and bipolaronic effects at low temperature regime with negligible electronic contribution. A considerably high power factor (∼ 100 μW/mK2) is obtained at room temperature.

Phase pure Calcium cobaltite is prepared by sol-gel method using citric acid as complexing agent. Optical properties are characterized by FTIR and UV-vis diffuse reflectance spectroscopy. Magnetic measurement at 5-390 shows presence of ferromagnetic to paramagnetic transition around 20 K.

Magnetic interactions in Gadolinium orthochromites (GdCrO3) are quite complex. It shows strong temperature dependency related to Gd3+-Gd3+, Gd3+-Cr3+ and Cr3+-Cr3+ interactions, resulting in exotic phenomena like spin flipping and spin reorientation. These behaviors are successfully explained by considering Cr 3d-Gd 4f magnetic coupling. The nearest neighbor symmetric and antisymmetric exchange coupling in Cr-sublattice was found to be Je = 11.058 K and D = 2.64 K from modified Curie-Weiss law modeled by Moriya, while positive Zeeman energy between net moments and the applied external magnetic fields was found to drive spin flipping.Magnetic interactions in Gadolinium orthochromites (GdCrO3) are quite complex. It shows strong temperature dependency related to Gd3+-Gd3+, Gd3+-Cr3+ and Cr3+-Cr3+ interactions, resulting in exotic phenomena like spin flipping and spin reorientation. These behaviors are successfully explained by considering Cr 3d-Gd 4f magnetic coupling. The nearest neighbor symmetric and antisymmetric exchange coupling in Cr-sublattice was found to be Je = 11.058 K and D = 2.64 K from modified Curie-Weiss law modeled by Moriya, while positive Zeeman energy between net moments and the applied external magnetic fields was found to drive spin flipping.

The joint effect of exchange and Rashba spin-orbit interactions is examined on the surface and quantum well states of ${\mathrm{Ag}}_{2}\mathrm{Bi}$-terminated Ag films grown on ferromagnetic Fe(110). The system displays a particular combination of time-reversal and translational symmetry breaking that strongly influences its electronic structure. Angle-resolved photoemission reveals asymmetric band-gap openings, due to spin-selective hybridization between Rashba-split surface states and exchange-split quantum well states. This results in an unequal number of states along positive and negative reciprocal space directions. We suggest that the peculiar asymmetry of the discovered electronic structure can have significant influence on spin-polarized transport properties.

Temperature dependent photoemission studies on lightly doped ( x = 0.025 ) sodium tungsten bronzes, NaxWO3 have been investigated by high-resolution photoemission spectroscopy. The experimental results show evidence for polaron formation at the valence band edge and the photoemission spectra taken in different modes of the electron analyzer suggest that the density of states at the valence band edge gradually moves to other k -points in the Brillouin zone with increasing temperature and explain the dynamics of polarons in the insulating disordered sodium tungsten bronzes.

Detailed magnetic studies of polycrystalline GdCrO3 show a spectrum of interesting features, such as temperature induced magnetization reversal, spin flipping and spin reorientation, etc, which arise due to the competing magnetic interactions within and between Cr and Gd-sublattices. It also exhibits a giant magnetocaloric effect (MCE) with a maximum entropy change of 36.97 J kg−1 K−1, an adiabatic temperature change of 19.12 K and a refrigeration capacity of 542 J kg−1 for a field change of 7 T at low temperatures. Such an exceptionally large MCE arises from the suppression of the spin entropy associated with the suppression of spin reorientation transition, in addition to the Gd-ordering, which makes it one of the best candidates for magnetic refrigeration among all known potential low temperature magnetic refrigerants.

In order to identify new oxide ion-conducting materials in the ceria family of oxides, the unique effect of co-doping is explored and a novel series of $Ce_{0.8}Sm_{0.2}_xCa_xO_{2-\delta}$ compositions is identified that have enhanced properties compared to the singledoped $Ce_{0.8}Sm_{0.2}O_{1.9}$ and $Ce_{0.8}Ca_{0.2}O_{1.9}$ compositions. Moreover, the superior characteristics of the co-doped $Ce_{0.8}Sm_{0.2-x}-Ca_xO_{2-\delta}$ powders prepared by the mixed-fuel process aid in obtaining 98% dense ceramics upon sintering at $1200^o$C for 6 h.Though a linear increase in conductivity is observed by replacing Sm with Ca, the composition with the maximum amount of Ca and the minimum amount of Sm exhibits a significant improvement in properties compared to the rest in the series. The composition $Ce_{0.80}Sm_{0.05}Ca_{0.15}O_{2-\delta}$ exhibits a conductivity as high as 1.22 \times $10^{-1} Scm^{-1}$ at $700^o$ C with minimum activation energy (0.56 eV) and a superior chemical stability to reduction compared to any of the hitherto known (CaSm) compositions. The absence of $Ce^{III}$, confirmed both from X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, strongly suggests that the observed increase in conductivity is solely due to the oxide ion conductivity and not due to the partial electronic contribution arising from the presence of $Ce^{III}$ and $Ce^{IV}$. To conclude, the experimental results on the $Ce_{0.8}Sm_{0.2-x}Ca_xO_{2-\delta}$ series underscore the unique effect of calcium co-doping in identifying a cost-effective new composition, with a remarkably high conductivity and enhanced chemical stability to reduction, for technological applications.

We report the results of the dc magnetization and ac susceptibility measurements on solid solutions of Ga2−xFexO3 (with x = 0.75, 1.0, and 1.25). Magnetic behavior in this series of compounds could broadly be explained by the molecular-field-approximation of a three-sublattice ferrimagnetic model considering three inequivalent octahedral sites. Analysis of frequency dispersion of ac susceptibility reveals a transition from the cluster-glass-like phase to the spin-glass-like phase with decreasing temperature for the x = 0.75 composition. Mentioned glassy behavior is found to gradually evolve with the composition (x) from the Ising type character to Heisenberg type behavior to unconventional glassy behavior for the x = 1.25 composition. Ga2−xFexO3 can hence serve as an ideal system for modeling complex spin glasses.

The magnetic properties and magnetocaloric effect of polycrystalline GdAlO3 and GdMnO3 have been investigated to assess their potential usage as magnetic refrigerants at cryogenic temperatures. These compounds undergo antiferromagnetic transitions at low temperatures which are associated with the giant magnetic entropy change effect () ~40.9 J Kg K−1 under a field change of 0–9 T for GdAlO3 while the moderate effect of 18 J Kg K−1 is observed for polycrystalline GdMnO3. Though the relative cooling power of both the stated materials is similar however, the absence of magnetic and thermal hysteresis makes GdAlO3 a more efficient magnetic refrigerant than GdMnO3.

We report various high-energy electron spectroscopic data on sintered polycrystalline La1−xSrxMnO3 and La1−xSrxCoO3 with x=0.0, 0.1, 0.2, 0.3, and 0.4. The data presented consist of x-ray photoelectron transition metal 2p core level and valence band spectra, x-ray initiated transition metal L3VV spectra, Bremsstrahlung isochromat spectra, and soft x-ray absorption spectra at the oxygen K-edge. Substitution of trivalent La with divalent Sr introduces holes into the system, driving an insulator-to-metal transition along with magnetic transitions. The spectral changes with progressive hole doping reflect the evolution of electronic structure across the phase transition.

Adsorption of organic molecules on well-oriented single-crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultraviolet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as ``interface states'' having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate $sp$ plateau. On the other hand, much care must be taken in assigning adsorbate-induced features when these lie within the $d$-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large $\ensuremath{\pi}$-conjugated organic molecules.

International audience; By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows the synthesis of two distinct phases of bilayer graphene with different properties. The first phase has Bernal AS stacking with respect to the first graphene layer and displays weak vertical interaction and electron doping. The long-range ordered moire pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has an AA stacking sequence with respect to the first layer and displays weak structural and electronic modulation and p-doping. The linearly dispersing Dirac state reveals the nearly freestanding character of this novel second-layer phase.

We examined by two-dimensional photoemission band mapping the electronic structure of Au films epitaxially grown on an Ag(111) substrate. The very similar structural and electronic properties of the two metals make this system extremely unfavorable for the occurrence and observation of electron confinement effects. At variance with previous spectroscopic studies, we show that the electron reflectivity at the interface sustains the formation of well-defined $sp$-derived quantum well states (QWS) and weak quantum well resonance states (QWRS) in the Au layers. The character and degree of confinement of these states are analyzed and quantitatively related to the Au/Ag interface reflectivity on the basis of density functional theory (DFT) band structure calculations.

The possibility of modifying the intermolecular interactions of absorbed benzene-carboxylic acids from coordination to hydrogen bonding by changing their surface coverage is demonstrated through a combination of scanning tunnelling microscopy, X-ray photo-emission spectroscopy and density functional theory calculations.

We discuss the evolution of Infrared reflectivity at room temperature under various pressures (P) and Se alloying concentration in the strongly correlated NiS2−xSex pyrite. Measurements gave a complete picture of the optical response of the system on approaching the P-induced and Se-induced metallic state. A peculiar non-monotonic (V-shaped) volume dependence was found for the quasiparticle spectral weight of both pure and Se-doped compounds.

In this paper it is shown that angle-resolved photoemission performed using low-energy photons on an organic-metal interface allows to clearly distinguish genuine interface states from features of substrate photoelectrons diffracted by the molecular lattice. As a model system an ordered monolayer of Zn-phthalocyanine is used as a diffraction lattice to probe the electronic band structure of a Ag(110) substrate. Photoemission close to normal emission geometry reveals strongly dispersive features absent in the pristine substrate spectra. Density functional theory modeling helped identifying these as bulk $sp$ direct transitions undergoing surface-umklapp processes. The present results establish the important role of final-state diffraction effects in photoemission experiments at organic-inorganic interfaces.

Ambient-condition Raman spectra were collected in the strongly correlated NiS${}_{1\ensuremath{-}x}$Se${}_{x}$ pyrite ($0\ensuremath{\le}x\ensuremath{\le}1.2$). Two samples ($x=0$ and $x=0.55$) were studied as a function of pressure up to 10 GPa, and for the $x=0.55$ sample the pressure dependence of the infrared reflectivity was also measured (0--10 GPa). This gave a complete picture of the optical response of that system on approaching the metallic state both by application of pres-sure and/or by Se alloying, which corresponds to a volume expansion. A peculiar nonmonotonic (V-shaped) volume dependence was found for the quasiparticle spectral weight of both pure and Se-doped compounds. In the $x=0.55$ sample the vibrational frequencies of the chalcogen dimer show an anomalous volume dependence on entering the metallic phase. The abrupt softening observed, particularly significant for the Se-Se pair, indicates the relevant role of the softness of the Se-Se bond as previously suggested by theoretical calculations.

We measured the magnetoconductance of bare and SQRT(3)×SQRT(3)-Bi/Ag-terminated ultrathin Ag(111) films by the micro-four-point probe method as a function of the applied magnetic field. The experimental curves were analyzed by introducing the results of photoemission investigation and band-structure calculations into the Hikami-Larkin-Nagaoka formula, in order to derive the characteristic fields of the two systems. The formation of the Rashba-type surface alloy was found to reduce the spin-relaxation time in the ultrathin film significantly.

We monitored the $sp$-quantum-well states of Ag films on Pt(111) by angle-resolved photoemission in order to examine the electron transmission across the Ag/Pt interface. For thin layers up to 3.5 nm, the Ag states are characterized by broad quasiparticle peaks and a reversal of the parabolic curvature near the center of the surface Brillouin zone. Remarkable departures from the expected nearly-free-electronlike band dispersion persist in films of more than 14 nm thickness. First-principles calculations and symmetry analysis demonstrate that the observed anomalies in the spectroscopic data can be straightforwardly linked to variations in the Ag/Pt transmission coefficient in the energy-momentum space.

We report a systematic investigation of the electronic properties and correlation effects in a two-dimensional magnetic surface alloy Mn/Cu(110) using a combination of spectroscopic techniques ranging from the angle-resolved photoemission, x-ray photoelectron, and x-ray absorption spectroscopy. Our results show that increasing Mn concentration surprisingly induces a progressive charge redistribution on the Cu(110) substrate, increasing the electron occupancy of the Cu Shockley surface state. Furthermore, new Mn-induced surface bands are identified at the $\overline{X}$ symmetry point that are backfolded with respect to the Brillouin-zone boundary and expose a band gap in the occupied states. Distinct satellite features in both the core-level photoemission and absorption spectra for a wide range of Mn concentrations suggest the presence of strong electronic correlations and increased ordering of this alloy with respect to its other known counterparts, such as Mn/Cu(100) and Mn/Ni(110).

The electronic structure of ultrathin Ag(111) films covered with a square root(3) x square root(3)-Bi/Ag ordered alloy was investigated by means of spin- and angle-resolved photoemission spectroscopy. Surface-state (SS) bands, spin split by the Rashba interaction, selectively couple to the quantum-well state (QWS) bands, originally spin degenerate, in the metal film. Gaps are found to open between QWS and SS with parallel spins, while free-electron-like QWS dispersions are observed for antiparallel spin configurations. The present results demonstrate that in a nonmagnetic metal film the spin degeneracy of the valence levels can be lifted by hybridization with Rashba-type SS bands.

The stabilization of single Fe atoms in the nanopores of an ultrathin silica film grown on Mo(112) is demonstrated with scanning tunneling microscopy (STM) and density functional theory (DFT). The Fe atoms are able to penetrate the openings in the oxide surface and adsorb in two different binding configurations at the metal-oxide interface. In the energetically preferred site, the Fe stays monomeric even at temperatures above 300 K. In the second configuration that is adopted in 10% of the cases, surface atoms can be attached to the subsurface species, resulting in the formation of Fe surface clusters. The interfacial Fe atoms preserve their magnetic moment, as shown by a distinct Kondo-like response in STM conductance spectra and DFT calculations.

The metal to insulator transition in the charge-transfer NiS2-xSex compound has been investigated through infrared reflectivity. Measurements performed by applying pressure to pure NiS2 (lattice contraction) and by Se alloying (lattice expansion) reveal that in both cases an anomalous metallic state is obtained. We find that optical results are not compatible with the linear Se-alloying vs pressure-scaling relation previously established through transport, thus pointing out the substantially different microscopic origin of the two transitions.