Showing papers by "László Forró published in 2019"

Light-Emitting Electrochemical Cells of Single Crystal Hybrid Halide Perovskite with Vertically Aligned Carbon Nanotubes Contacts

Abstract: Based on the reported ion migration under an electric field in hybrid lead halide perovskites we have developed a bright, light-emitting electrochemical cell with CH3NH3PbBr3 single crystals direct...

Differential Response of the Photoluminescence and Photocurrent of Polycrystalline CH3NH3PbI3 and CH3NH3PbBr3 to the Exposure to Oxygen and Nitrogen

Abstract: Because of their excellent photoelectric properties, organic–inorganic metal halide perovskites (MHPs), such as methylammonium lead triiodide, CH3NH3PbI3 (MAPbI3), and methylammonium lead tribromid...

Characterizing the maximum number of layers in chemically exfoliated graphene

Abstract: An efficient route to synthesize macroscopic amounts of graphene is highly desired and bulk characterization of such samples, in terms of the number of layers, is equally important. We present a Raman spectroscopy-based method to determine the typical upper limit of the number of graphene layers in chemically exfoliated graphene. We utilize a controlled vapour-phase potassium intercalation technique and identify a lightly doped stage, where the Raman modes of undoped and doped few-layer graphene flakes coexist. The spectra can be unambiguously distinguished from alkali doped graphite, and modeling with the typical upper limit of the layers yields an upper limit of flake thickness of five layers with a significant single-layer graphene content. Complementary statistical AFM measurements on individual few-layer graphene flakes find a consistent distribution of the layer numbers.

Preferential out-of-plane conduction and quasi-one-dimensional electronic states in layered van der Waals material 1T-TaS2

Abstract: Layered metallic transition-metal dichalcogenides are conventionally seen as two-dimensional conductors, despite a scarcity of systematic studies of the interlayer charge transport conducted in a well-defined geometry. Here, we reveal c-axis-oriented quasi-one-dimensional electronic states in 1T-TaS2, a layered system hosting a plethora of diverse phases, by probing its out-of-plane electrical resistivity using focused-ion-beam-tailored crystals. While the in-plane conduction appears non-metallic due to presence of a unique nanoarray of charge density wave (CDW) domains, the interlayer resistivity has a metallic temperature dependence with the anisotropy close to one, as a result of intertwining of the orbital and CDW orders. The compound is known for what was believed to be a Mott-type localisation below 150 K, yet with our data, supplemented by ab-initio calculations, we interpret the transition as a Peierls-like instability of the quasi-one-dimensional electronic structure. Our findings present a very unintuitive behaviour in a van der Waals crystal.

Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe3O4 film on SrTiO3:Nb by v-MOKE and ferromagnetic resonance.

Abstract: The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe3O4) thin film grown by infrared pulsed-laser deposition on SrTiO3(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, TV, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below TV by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > TV and 〈110〉 for T < TV. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe3O4 thin films by proper control on the growth parameters and substrate choice.

Pressure-induced transformation of CH3NH3PbI3: the role of the noble-gas pressure transmitting media

Abstract: The photovoltaic perovskite, methyl­ammonium lead triiodide [CH3NH3PbI3 (MAPbI3)], is one of the most efficient materials for solar energy conversion. Various kinds of chemical and physical modifications have been applied to MAPbI3 towards better understanding of the relation between composition, structure, electronic properties and energy conversion efficiency of this material. Pressure is a particularly useful tool, as it can substantially reduce the interatomic spacing in this relatively soft material and cause significant modifications to the electronic structure. Application of high pressure induces changes in the crystal symmetry up to a threshold level above which it leads to amorphization. Here, a detailed structural study of MAPbI3 at high hydro­static pressures using Ne and Ar as pressure transmitting media is reported. Single-crystal X-ray diffraction experiments with synchrotron radiation at room temperature in the 0–20 GPa pressure range show that atoms of both gaseous media, Ne and Ar, are gradually incorporated into MAPbI3, thus leading to marked structural changes of the material. Specifically, Ne stabilizes the high-pressure phase of NexMAPbI3 and prevents amorphization up to 20 GPa. After releasing the pressure, the crystal has the composition of Ne0.97MAPbI3, which remains stable under ambient conditions. In contrast, above 2.4 GPa, Ar accelerates an irreversible amorphization. The distinct impacts of Ne and Ar are attributed to differences in their chemical reactivity under pressure inside the restricted space between the PbI6 octahedra.

Persistent antiferromagnetic order in heavily overdoped Ca1-x La x FeAs2.

Abstract: In the Ca1−x La x FeAs2 (1 1 2) family of pnictide superconductors, we have investigated a highly overdoped composition (x = 0.56), prepared by a high-pressure, high-temperature synthesis. Magnetic measurements show an antiferromagnetic transition at T N = 120 K, well above the one at lower doping (0.15 < x < 0.27). Below the onset of long-range magnetic order at T N, the electrical resistivity is strongly reduced and is dominated by electron–electron interactions, as evident from its temperature dependence. The Seebeck coefficient shows a clear metallic behavior as in narrow band conductors. The temperature dependence of the Hall coefficient and the violation of Kohler's rule agree with the multiband character of the material. No superconductivity was observed down to 1.8 K. The success of the high-pressure synthesis encourages further investigations of the so far only partially explored phase diagram in this family of Iron-based high temperature superconductors.

Persistent antiferromagnetic order in heavily overdoped Ca$_{1-x}$La$_x$FeAs$_2$

Abstract: In the Ca$_{1-x}$La$_x$FeAs$_2$ (112) family of pnictide superconductors, we have investigated a highly overdoped composition (x = 0.56), prepared by high-pressure, high-temperature synthesis. Magnetic measurements show an antiferromagnetic transition at TN = 120K, well above the one at lower doping (0.15 < x < 0.27). Below the onset of long-range magnetic order at TN, the electrical resistivity is strongly reduced and is dominated by electron-electron interactions, as evident from its temperature dependence. The Seebeck coefficient shows a clear metallic behavior as in narrow band conductors. The temperature dependence of the Hall coefficient and the violation of Kohler's rule agree with the multiband character of the material. No superconductivity was observed down to 1.8 K. The success of the high-pressure synthesis encourages further investigations of the so far only partially explored phase diagram in this family of Iron-based high temperature superconductors.

Iron-Selenide Phenomenology in Superconducting Pr$_{4}$Fe$_{2}$As$_2$Te$_{0.88}$O$_4$

Abstract: The unified modeling of iron-based high-temperature superconductors has been complicated by different phenomenologies in the iron-pnictides and iron-chalcogenides. Here we use x-ray diffraction and angle-resolved photoemission spectroscopy to investigate the lattice and electronic properties of the Pr$_{4}$Fe$_{2}$As$_2$Te$_{0.88}$O$_4$ ($T_c=25$ K) superconductor. The crystal structure undergoes a high-temperature ($T_s = 250$ K) orthorhombic transition with no apparent magnetic transition. Further, we find the Fermi surface to consist of zone-corner electron pockets only. In addition to superconductivity, a second electronic instability -- evidenced by a spectroscopic gap below $T_g\approx 2\times T_c$ -- is found. Both, the electronic structure and the sequence of transitions, resemble what has been reported in monolayer and bulk FeSe. Our results thus provide a direct link between the iron-pnictide and iron-chalcogenide families of superconductors.

Decoupling of Lattice and Orbital Degrees of Freedom in an Iron-Pnictide Superconductor.

Abstract: The interplay of structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity, defined as the breaking of four-fold symmetry triggered by electronic degrees of freedom, is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction (XRD), we reveal a new structural orthorhombic phase with an exceptionally high onset temperature ($T_\mathrm{ort} \sim 250$ K), which coexists with superconductivity ($T_\mathrm{c} = 25$ K), in an electron-doped iron-pnictide superconductor far from the underdoped region. Furthermore, our angle-resolved photoemission spectroscopy (ARPES) measurements demonstrate the absence of electronic nematic order as the driving mechanism, in contrast to other underdoped iron pnictides where nematicity is commonly found. Our results establish a new, high temperature phase in the phase diagram of iron-pnictide superconductors and impose strong constraints for the modeling of their superconducting pairing mechanism.