Showing papers by "László Forró published in 2016"
TL;DR: In this article, the superconducting properties of NbSe2 as it approaches the monolayer limit are investigated by means of magnetotransport measurements, uncovering evidence of spin-momentum locking.
Abstract: The superconducting properties of NbSe2 as it approaches the monolayer limit are investigated by means of magnetotransport measurements, uncovering evidence of spin–momentum locking.
888 citations
TL;DR: In this article, the authors demonstrate reversible tuning of superconductivity and charge-density-wave (CDW) order in model 2D TMD metal NbSe_{2} by an ionic liquid gate.
Abstract: Recent experimental advances in atomically thin transition metal dichalcogenide (TMD) metals have unveiled a range of interesting phenomena including the coexistence of charge-density-wave (CDW) order and superconductivity down to the monolayer limit. The atomic thickness of two-dimensional (2D) TMD metals also opens up the possibility for control of these electronic phase transitions by electrostatic gating. Here, we demonstrate reversible tuning of superconductivity and CDW order in model 2D TMD metal NbSe_{2} by an ionic liquid gate. A variation up to ∼50% in the superconducting transition temperature has been observed. Both superconductivity and CDW order can be strengthened (weakened) by increasing (reducing) the carrier density in 2D NbSe_{2}. The doping dependence of these phase transitions can be understood as driven by a varying electron-phonon coupling strength induced by the gate-modulated carrier density and the electronic density of states near the Fermi surface.
160 citations
TL;DR: The synthesis of a ferromagnetic photovoltaic CH3NH3(Mn:Pb)I3 material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman–Kittel–Kasuya–Yosida interactions without heating up the spin system is reported.
Abstract: The demand for ever-increasing density of information storage and speed of manipulation boosts an intense search for new magnetic materials and novel ways of controlling the magnetic bit. Here, we report the synthesis of a ferromagnetic photovoltaic CH3NH3(Mn:Pb)I-3 material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman-Kittel-Kasuya-Yosida interactions without heating up the spin system. Our finding offers an alternative, very simple and efficient way of optical spin control, and opens an avenue for applications in low-power, light controlling magnetic devices.
112 citations
TL;DR: The effects of CH3NH3PbI3 photovoltaic perovskites in human lung adenocarcinoma epithelial cells, human dopaminergic neuroblastoma cells and murine primary hippocampal neurons are reported by using multiple assays and electron microscopy studies.
Abstract: New technologies launch novel materials; besides their performances in products, their health hazards must be tested. This applies to the lead halide perovskite CH3NH3PbI3 as well, which offers fulgurate applications in photovoltaic devices. We report the effects of CH3NH3PbI3 photovoltaic perovskites in human lung adenocarcinoma epithelial cells (A549), human dopaminergic neuroblastoma cells (SH-SY5Y) and murine primary hippocampal neurons by using multiple assays and electron microscopy studies. In cell culture media the major part of the dissolved CH3NH3PbI3 has a strong cell-type dependent effect. Hippocampal primary neurons and neuroblastoma cells suffer a massive apoptotic cell death, whereas exposure to lung epithelial cells dramatically alters the kinetics of proliferation, metabolic activity and cellular morphology without inducing noticeable cell death. Our findings underscore the critical importance of conducting further studies to investigate the effect of short and long-term exposure to CH3NH3PbI3 on health and environment.
108 citations
TL;DR: In this paper, the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH3NH3PbI3 in open nanofluidic channels is reported.
Abstract: Spatial positioning of nanocrystal building blocks on a solid surface is a prerequisite for assembling individual nanoparticles into functional devices. Here, we report on the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH3NH3PbI3 in open nanofluidic channels. The guided growth, visualized in real-time with a simple optical microscope, undergoes through a metastable solvatomorph formation in polar aprotic solvents. The presently discovered crystallization leads to the fabrication of mm2-sized surfaces composed of perovskite nanowires having controlled sizes, cross-sectional shapes, aspect ratios and orientation which have not been achieved thus far by other deposition methods. The automation of this general strategy paves the way towards fabrication of wafer-scale perovskite nanowire thin films well-suited for various optoelectronic devices, e.g. solar cells, lasers, light-emitting diodes and photodetectors.
83 citations
TL;DR: The authors' angle-resolved photoemission spectroscopy measurements performed on the β-phase of quasi-one-dimensional bismuth iodide Bi4I4 reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
Abstract: Recent progress in the field of topological states of matter(1,2) has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs)(3-6), followed by closely related ternary compounds(7-16) and predictions of several weak TIs(17-19). However, both the conceptual richness of Z$_2$ classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z$_2$ topological insulator is theoretically predicted and experimentally confirmed in the $\beta$-phase of quasi-one-dimensional bismuth iodide Bi$_4$I$_4$. The electronic structure of $\beta$-Bi$_4$I$_4$, characterized by Z$_2$ invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
82 citations
TL;DR: In this article, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi4I4.
Abstract: Recent progress in the field of topological states of matter has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs ,,,), followed by closely related ternary compounds and predictions of several weak TIs (refs ,,). However, both the conceptual richness of Z2 classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of β-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
72 citations
TL;DR: In this paper, the graphoepitaxial liquid-solid growth of perovskite nanowires of the photovoltaic compound CH$(3$NH$_3$PbI$_ 3$ in open nanofluidic channels is reported.
Abstract: Spatial positioning of nanocrystal building blocks on a solid surface is a prerequisite for assembling individual nanoparticles into functional devices. Here, we report on the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH$_3$NH$_3$PbI$_3$ in open nanofluidic channels. The guided growth, visualized in real-time with a simple optical microscope, undergoes through a metastable solvatomorph formation in polar aprotic solvents. The presently discovered crystallization leads to the fabrication of mm2-sized surfaces composed of perovskite nanowires having controlled sizes, cross-sectional shapes, aspect ratios and orientation which have not been achieved thus far by other deposition methods. The automation of this general strategy paves the way towards fabrication of wafer-scale perovskite nanowire thin films well-suited for various optoelectronic devices, e.g. solar cells, lasers, light-emitting diodes and photodetectors.
70 citations
TL;DR: In this paper, single crystals of WS2 were prepared by chemical vapor transport method using sulfur as a transport agent and measurements of electrical resistivity, Seebeck coefficient and thermal conductivity as a function of temperature along the in-plane and the out-of-plane directions reveal distinct trends.
52 citations
TL;DR: In this paper, a field effect phototransistor based on individual carbon nanotubes (CNTs) sensitized by CH3NH3PbII3 nanowires (MAPbI3NWs) was constructed, which achieved light responsivities of R = 7.7 × 10(5) A W(-1) under low-lighting conditions in the nW mm(-2) range.
Abstract: Field-effect phototransistors were fabricated based on individual carbon nanotubes (CNTs) sensitized by CH3NH3PbI3 nanowires (MAPbI3NWs). These devices represent light responsivities of R = 7.7 × 10(5) A W(-1) under low-lighting conditions in the nW mm(-2) range, unprecedented among CNT-based photodetectors. At high incident power (∼1 mW mm(-2)), light soaking results in a negative photocurrent, turning the device insulating. We interpret the phenomenon as a result of efficient free photoexcited charge generation and charge transfer of photoexcited holes from the perovskite to the carbon nanotube. The charge transfer improves conductance by increasing the number of carriers, but leaves electrons behind. At high illumination intensity their random electrostatic potential quenches mobility in the nanotube.
50 citations
TL;DR: It is shown that (I and (II) are characterized by an identical tetragonal unit cell but different space groups: I422 for (I) and P42212 for (II).
Abstract: The crystal structure of the pristine (I) and aged (II) crystals of CH3NH3PbI3 (hereafter MAPbI3) hybrid organic–inorganic lead iodide has been studied at 293 K with high-precision single-crystal X-ray diffraction using a synchrotron light source. We show that (I) and (II) are characterized by an identical tetragonal unit cell but different space groups: I422 for (I) and P42212 for (II). Both space groups are subgroups of I4/mcm, which is widely used for MAPbI3. The main difference between (I) and (II) comes from the difference in hydrogen bonds between the MA+ cation and the PbI3 framework which is the direct consequence of H2O insertion in the aged crystal (II).
TL;DR: Conduction electron spin–lattice and spin–spin relaxation times of 175 ns at 300 K in 37±7 nm carbon spheres are reported, which is remarkably long for any conducting solid-state material of comparable size.
Abstract: The time-window for processing electron spin information (spintronics) in solid-state quantum electronic devices is determined by the spin–lattice and spin–spin relaxation times of electrons. Minimizing the effects of spin–orbit coupling and the local magnetic contributions of neighbouring atoms on spin–lattice and spin–spin relaxation times at room temperature remain substantial challenges to practical spintronics. Here we report conduction electron spin–lattice and spin–spin relaxation times of 175 ns at 300 K in 37±7 nm carbon spheres, which is remarkably long for any conducting solid-state material of comparable size. Following the observation of spin polarization by electron spin resonance, we control the quantum state of the electron spin by applying short bursts of an oscillating magnetic field and observe coherent oscillations of the spin state. These results demonstrate the feasibility of operating electron spins in conducting carbon nanospheres as quantum bits at room temperature. Electronic decoherence due to spin-orbit and magnetic interactions limits the application of spintronic nanosystems in quantum information processing. Here, the authors report notably long spin-lattice and spin-spin relaxation times of 175 ns at room temperature in carbon nanospheres.
TL;DR: In this article, the anatase/rutile ratio of the TiO2 in the composite can be tuned by the manipulation of acid concentration used for the hydrothermal crystallization.
Abstract: Titanium dioxide/multiwall carbon nanotube (TiO2/MWCNT) composites were prepared by sol–gel method with hydrolysis of titanium alkoxide precursor followed by crystallization via hydrothermal treatment at different acid concentrations. Transmission electron microscopy (TEM) measurements revealed that the acid concentration has serious control over the morphology which is in good agreement with the change of specific surface area. According to the X-ray diffraction analysis of the samples the anatase/rutile ratio of the TiO2 in the composite can be tuned by the manipulation of acid concentration used for the hydrothermal crystallization. Samples were tested in aqueous-phase photocatalytic degradation reaction of salicylic acid in a batch-type foam reactor using relatively low power UV-A irradiation. The change in salicylic acid concentration was monitored by UV–vis spectroscopy technique. Anatase/rutile ratio decreased with increasing acid concentration and photocatalytic performance was decreased with increasing rutile content, however low rutile content (11 wt%) enhanced the photocatalytic efficiency above all prepared samples.
TL;DR: In this article, the experimental and theoretical state-of-the-art concerning the electron spin dynamics in graphene, silicene, phosphorene, transition metal dichalcogenides, covalent heterostructures of organic molecules and topological materials are reviewed.
Abstract: The growing library of two-dimensional layered materials is providing researchers with a wealth of opportunity to explore and tune physical phenomena at the nanoscale. Here, we review the experimental and theoretical state-of-art concerning the electron spin dynamics in graphene, silicene, phosphorene, transition metal dichalcogenides, covalent heterostructures of organic molecules and topological materials. The spin transport, chemical and defect induced magnetic moments, and the effect of spin-orbit coupling and spin relaxation, are also discussed in relation to the field of spintronics.
TL;DR: An implementation of photonic force microscopy (PFM) is reported on to explore the Brownian motion and optical trappability of monocrystalline potassium niobate (KNbO3) nano/microsized particles having sizes within the range of 50 to 750 nm.
Abstract: Perovskite alkaline niobates, due to their strong nonlinear optical properties, including birefringence and the capability to produce second-harmonic generation (SHG) signals, attract a lot of attention as potential candidates for applications as local nano/microsized mechano-optical probes. Here, we report on an implementation of photonic force microscopy (PFM) to explore the Brownian motion and optical trappability of monocrystalline potassium niobate (KNbO3) nano/microsized particles having sizes within the range of 50 to 750 nm. In particular, we exploit the anisotropic translational diffusive regime of the Brownian motion to quantify thermal fluctuations and optical forces of singly-trapped KNbO3 particles within the optical trapping volume of a PFM microscope. We also show that, under near-infrared (NIR) excitation of the highly focused laser beam of the PFM microscope, a single optically-trapped KNbO3 particle reveals a strong SHG signal manifested by a narrow peak (λem = 532 nm) at half the excitation wavelength (λex = 1064 nm). Moreover, we demonstrate that the thus induced SHG emission can be used as a local light source that is capable of optically exciting molecules of an organic dye, Rose Bengal (RB), which adhere to the particle surface, through the mechanism of luminescence energy transfer (LET).
TL;DR: In this paper, a detailed study of the interplay between the different interactions which appear in the Brownian motion of a micronsized sphere immersed in a viscoelastic fluid measured with optical trapping interferometry is provided.
Abstract: We provide a detailed study of the interplay between the different interactions which appear in the Brownian motion of a micronsized sphere immersed in a viscoelastic fluid measured with optical trapping interferometry. To explore a wide range of viscous, elastic, and optical forces, we analyze two different viscoelastic solutions at various concentrations, which provide a dynamic polymeric structure surrounding the Brownian sphere. Our experiments show that, depending on the fluid, optical forces, even if small, slightly modify the complex modulus at low frequencies. Based on our findings, we propose an alternative methodology to calibrate this kind of experimental set-up when non-Newtonian fluids are used. Understanding the influence of the optical potential is essential for a correct interpretation of the mechanical properties obtained by optically-trapped probe-based studies of biomaterials and living matter.
TL;DR: This work synthesized the perovskite solutions and discovered the graphoepitaxial nanowire growth and discussed the results and implications at all stages.
Abstract: Scientific Reports 6: Article number: 1983410.1038/srep19834; published online: January252016; updated: February252016.
Balint Nafradi was omitted from the author list in the original version of this Article. This has been corrected in the PDF and HTML versions of the Article.
The Author Contributions section now reads:
L.F. initiated the research. E.H. synthesized the perovskite solutions and discovered the graphoepitaxial nanowire growth. M.S. and E.B. prepared the microfabricated devices. M.S., E.B. and B.N. performed the photocurrent measurements and analyzed the data. A.S. performed and analyzed the fluorescence measurements. E.H., A.S., M.S., E.B., B.N. and L.F. discussed the results and implications and commented on the manuscript at all stages.
TL;DR: In this article, a simple, rapid, and calibration-free method for reading the thickness directly from the color map of nanowire perovskite films seen in standard optical microscope with visible light is presented.
Abstract: Hybrid halide perovskite photovoltaic materials show a remarkable light conversion efficiency in various optoelectronic devices. In the fabrication of these solar cells, light emitting diodes, laser and photodetector prototypes the thickness of the perovskite is an important parameter since the light is absorbed within a thin layer of a few hundred nanometers. Nevertheless, making perovskite coatings with various solution-based and evaporation methods showing highly reproducible thickness and area coverage is still an issue. Therefore, rapid and reliable quality-control of the film morphology is needed. This report shows a simple, rapid, and calibration-free method for reading the thickness directly from the color map of nanowire perovskite films seen in standard optical microscope with visible light. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Patent•
17 Nov 2016TL;DR: In this article, a magnetic-photoconductive material including orientable magnetic moments or spins is presented, the material being configured to generate photo-carriers permitting to orientate or re-orientate the magnetic moments at a material temperature less than the Curie Temperature (TC) or Curie point.
Abstract: The present invention concerns a magnetic-photoconductive material including orientable magnetic moments or spins, the material being configured to generate photo-carriers permitting to orientate or re-orientate the magnetic moments or spins at a material temperature less than the Curie Temperature (TC) or Curie point.
01 Sep 2016
TL;DR: A novel reflection amplifier circuit consisting of three graphene FETs (GFETs) which leverages its unique NDR characteristics and it is shown through rigorous simulation and modeling that broadband amplification exceeding several hundreds of GHz should be possible for the scaled graphene circuit.
Abstract: While RF transistor amplifiers—such as the field effect transistor (FET) amplifier which leverages its transconductance for amplification—are the key enablers of signal amplification in today's wireless communication; their ability to provide amplification degrades with increasing frequencies, thereby requiring multiple amplification stages which makes the device noisy, expensive and bigger in size. Owing to their broadband amplification capabilities, reflection-type amplifiers based on negative differential resistance (NDR) devices provide means to overcome these limitations. Herein, we propose a novel reflection amplifier circuit consisting of three graphene FETs (GFETs) which leverages its unique NDR characteristics. We show through rigorous simulation and modeling that broadband amplification exceeding several hundreds of GHz should be possible for the scaled graphene circuit. In addition, both the gain and frequency of operation can be highly modulated by varying the bias in the NDR region. Finally, we provide an experimental evidence of reflection amplification in the proposed circuit.
TL;DR: In this paper, an anisotropic spin-orbit Hamiltonian was analyzed in the framework of the Elliott-Yafet theory of spin-relaxation in metals and the analysis provided an experimental input for the first-principles theories of spinorbit interaction in layered carbon and thus to a better understanding of spin relaxation phenomena in graphene and in other layered materials.
Abstract: We report Electron Spin Resonance (ESR) measurements on stage-I potassium intercalated graphite (KC$_8$). Angular dependent measurements show that the spin-lattice relaxation time is longer when the magnetic field is perpendicular to the graphene layer as compared to when the magnetic field is in the plane. This anisotropy is analyzed in the framework of the Elliott-Yafet theory of spin-relaxation in metals. The analysis considers an anisotropic spin-orbit Hamiltonian and the first order perturbative treatment of Elliott is reproduced for this model Hamiltonian. The result provides an experimental input for the first-principles theories of spin-orbit interaction in layered carbon and thus to a better understanding of spin-relaxation phenomena in graphene and in other layered materials as well.
TL;DR: In this paper, electron spin resonance (ESR) measurements on stage-I potassium intercalated graphite (KC8) were performed and it was shown that the spin-lattice relaxation time is longer when the magnetic field is perpendicular to the graphene layer as compared to when the field is in the plane.
Abstract: We report electron spin resonance (ESR) measurements on stage-I potassium intercalated graphite (KC8). Angular dependent measurements show that the spin-lattice relaxation time is longer when the magnetic field is perpendicular to the graphene layer as compared to when the magnetic field is in the plane. This anisotropy is analyzed in the framework of the Elliott-Yafet theory of spin-relaxation in metals. The analysis considers an anisotropic spin-orbit Hamiltonian and the first order perturbative treatment of Elliott is reproduced for this model Hamiltonian. The result provides an experimental input for the first-principles theories of spin-orbit interaction in layered carbon and thus to a better understanding of spin-relaxation phenomena in graphene and in other layered materials as well.
TL;DR: The resistivity shows saturation at high temperatures, suggesting that the material approaches the Mott-Ioffe-Regel limit for metallic conduction, and the pressure and temperature dependence of the in-plane resistivity is studied.
Abstract: We present a detailed study of the electrical transport properties of a recently discovered iron-based superconductor: Sm4Fe2As2Te0.72O2.8F1.2. We followed the temperature dependence of the upper critical field by resistivity measurement of single crystals in magnetic fields up to 16 T, oriented along the two main crystallographic directions. This material exhibits a zero-temperature upper critical field of 90 T and 65 T parallel and perpendicular to the Fe2As2 planes, respectively. An unprecedented superconducting magnetic anisotropy gamma(H) = H-c2(ab)/H-c2(c) similar to 14 is observed near T-c, and it decreases at lower temperatures as expected in multiband superconductors. Direct measurement of the electronic anisotropy was performed on microfabricated samples, showing a value of rho(c)/rho(ab) (300 K) similar to 5 that rises up to 19 near T-c. Finally, we have studied the pressure and temperature dependence of the in-plane resistivity. The critical temperature decreases linearly upon application of hydrostatic pressure (up to 2 GPa) similarly to overdoped cuprate superconductors. The resistivity shows saturation at high temperatures, suggesting that the material approaches the Mott-Ioffe-Regel limit for metallic conduction. Indeed, we have successfully modelled the resistivity in the normal state with a parallel resistor model that is widely accepted for this state. All the measured quantities suggest strong pressure dependence of the density of states.
TL;DR: In this paper, a detailed study of the interplay between the different interactions which appear in the Brownian motion of a micronsized sphere immersed in a viscoelastic fluid measured with optical trapping interferometry is provided.
Abstract: We provide a detailed study of the interplay between the different interactions which appear in the Brownian motion of a micronsized sphere immersed in a viscoelastic fluid measured with optical trapping interferometry. To explore a wide range of viscous, elastic and optical forces, we analyze two different viscoelastic solutions at various concentrations, which provide a dynamic polymeric structure surrounding the Brownian sphere. Our experiments show that, depending of the fluid, optical forces, even if small, slightly modify the complex modulus at low frequencies. Based on our findings, we propose an alternative methodology to calibrate this kind of experimental set-up when non-Newtonian fluids are used. Understanding the influence of the optical potential is essential for a correct interpretation of the mechanical properties obtained by optically-trapped probe-based studies of biomaterials and living matter.
10 Jul 2016
TL;DR: In this article, the authors demonstrate the use of back focal plane interferometry for the measurement of the forces felt by a trapped sphere moving inside a hollow photonic crystal cavity.
Abstract: We demonstrate the use of back focal plane interferometry for the measurement of the forces felt by a trapped sphere moving inside a hollow photonic crystal cavity. The presence of two trapping regimes, one of these being a back-action effect, is observable in concordance to previous observations. We also demonstrate that particle can be differentiated by size with the monitoring of the light transmitted through the access waveguide, which is modulated via the particle induce resonance shift.