L. Forro - Institute of Physics of Complex Matter, FBS, Swiss Federal Institute of Technology

L. Forro
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L. Forro
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Institute of Physics of Complex Matter, FBS, Swiss Federal Institute of Technology
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Physics - Strongly Correlated Electrons (25)
 
Physics - Materials Science (22)
 
Physics - Mesoscopic Systems and Quantum Hall Effect (9)
 
Physics - Superconductivity (8)
 
Physics - Soft Condensed Matter (4)
 
Physics - Statistical Mechanics (2)
 
Physics - Disordered Systems and Neural Networks (1)
 
Physics - Biological Physics (1)

Publications Authored By L. Forro

We report magnetic and thermodynamic properties of a $4d^1$ (Mo$^{5+}$) magnetic insulator MoOPO$_4$ single crystal, which realizes a $J_1$-$J_2$ Heisenberg spin-$1/2$ model on a stacked square lattice. The specific-heat measurements show a magnetic transition at 16 K which is also confirmed by magnetic susceptibility, ESR, and neutron diffraction measurements. Magnetic entropy deduced from the specific heat corresponds to a two-level degree of freedom per Mo$^{5+}$ ion, and the effective moment from the susceptibility corresponds to the spin-only value. Read More

2017May
Affiliations: 1Institute of Physics, Zagreb, Croatia, 2Institute of Physics, Zagreb, Croatia, 3Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia, 41. Physikalisches Institut, Universität Stuttgart, Stuttgart, Germany, 5Ecole Polytechnique Fédérale de Lausanne, LPMC, Lausanne, Switzerland, 6Ecole Polytechnique Fédérale de Lausanne, LPMC, Lausanne, Switzerland, 71. Physikalisches Institut, Universität Stuttgart, Stuttgart, Germany, 8Institute of Physics, Zagreb, Croatia, 9Institute of Physics, Zagreb, Croatia

The controversy regarding the precise nature of the high-temperature phase of 1T-TiSe2 lasts for decades. It has intensified in recent times when new evidence for the excitonic origin of the low-temperature charge-density wave state started to unveil. Here we address the problem of the high-temperature phase through precise measurements and detailed analysis of the optical response of 1T-TiSe2 single crystals. Read More

We present results from light scattering experiments on three insulating antiferromagnetic cuprates, YBa$_2$Cu$_3$O$_{6.05}$, Bi$_2$Sr$_2$YCu$_2$O$_{8+\delta}$, and La$_2$CuO$_4$ as a function of polarization and excitation energy. The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of the excitation energy. Read More

We report a detailed study of the transport coefficients of $\beta$-Bi$_4$I$_4$ quasi-one dimensional topological insulator. Electrical resistivity, thermoelectric power, thermal conductivity and Hall coefficient measurements are consistent with the possible appearance of a charge density wave order at low temperatures. Both electrons and holes contribute to the conduction in $\beta$-Bi$_4$I$_4$ and the dominant type of charge carrier changes with temperature as a consequence of temperature-dependent carrier densities and mobilities. Read More

We report a detailed study of the electrical transport properties of single crystals of Pr$_4$Fe$_2$As$_2$Te$_{1-x}$O$_4$, a recently discovered iron-based superconductor. Resistivity, Hall effect and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohler's law indicate multiband effects in this compound. Read More

We present a detailed study of the electrical transport properties of a recently discovered iron-based superconductor: Sm$_4$Fe$_2$As$_2$Te$_{0.72}$O$_{2.8}$F$_{1. Read More

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. Read More

Methylammonium lead iodide perovskite has revolutionized the field of third generation solid-state solar cells leading to simple solar cell structures1 and certified efficiencies up to 20.1%. Recently the peculiar light harvesting properties of organometal halide perovskites have been exploited in photodetectors where responsivities of ~3. Read More

Field-effect phototransistors were fabricated based on individual carbon nanotubes (CNTs) sensitized by CH$_3$NH$_3$PbI$_3$ nanowires (MAPbI$_3$NW). These devices represent light responsivities of R=7.7x10$^5$ A/W at low-lighting conditions in the nWmm$^{-2}$ range, unprecedented among CNT-based photo detectors. Read More

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 CH$_3$NH$_3$(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. Read More

The time-window for processing electron spin information (spintronics) in solid-state quantum electronic devices is determined by the spin-lattice (T1) and spin-spin (T2) relaxation times of electrons. Minimising the effects of spin-orbit coupling and the local magnetic contributions of neighbouring atoms on T1 and T2 at room temperature remain substantial challenges to practical spintronics. Here, we report a record-high conduction electron T1=T2 of 175 ns at 300 K in 37 nm +/- 7 nm carbon spheres, which exceeds by far the highest values observed for any conducting solid state material of comparable size. Read More

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. Read More

The $1/4$-filled organic compound, $\delta$-(EDT-TTF-CONMe$_{2}$)$_{2}$AsF$_6$ is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. Read More

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. Read More

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. Read More

Albeit difficult to access experimentally, the density of states (DOS) is a key parameter in solid state systems which governs several important phenomena including transport, magnetism, thermal, and thermoelectric properties. We study DOS in an ensemble of potassium intercalated single-wall carbon nanotubes (SWCNT) and show using electron spin resonance spectroscopy that a sizeable number of electron states are present, which gives rise to a Fermi-liquid behavior in this material. A comparison between theoretical and the experimental DOS indicates that it does not display significant correlation effects, even though the pristine nanotube material shows a Luttinger-liquid behavior. Read More

We report the synthesis of single crystals of a novel layered iridate Ba$_{21}$Ir$_9$O$_{43}$, and present the crystallographic, transport and magnetic properties of this material. The compound has a hexagonal structure with two iridium oxide layers stacked along the $c$ direction. One layer consists of a triangular arrangement of Ir$_2$O$_9$ dimers while the other layer comprises two regular octahedra and one triangular pyramid, forming inter-penetrated triangular lattices. Read More

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$_4$. Read More

Since the discovery of its photovoltaic properties organometallic salt CH$_3$NH$_3$PbI$_3$ became the subject of vivid interest. The material exhibits high light conversion efficiency, it lases in red color, and it can serve as the basis for light emitting diodes and photodetectors. Here we report another surprising feature of this material family, the photo-tunability of the diode response of a heterojunction made of CH$_3$NH$_3$PbI$_3$ and its close relative, CH$_3$NH$_3$SnI$_3$. Read More

The intrinsic d.c. electrical resistivity ($\rho$) - measurable on single crystals only - is often the quantity first revealing the properties of a given material. Read More

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. Read More

Two-dimensional transition metal dichalcogenides with strong spin-orbit interactions and valley-dependent Berry curvature effects have attracted tremendous recent interests. Although novel single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied, effects of spin-momentum locking on collective quantum phenomena remain unexplored. Here we report an observation of superconducting monolayer NbSe$_2$ with an in-plane upper critical field over six times of the Pauli paramagnetic limit by magneto-transport measurements. Read More

Two-dimensional (2D) atomic materials possess very different properties from their bulk counterparts. While changes in the single-particle electronic properties have been extensively investigated, modifications in the many-body collective phenomena in the exact 2D limit, where interaction effects are strongly enhanced, remain mysterious. Here we report a combined optical and electrical transport study on the many-body collective-order phase diagram of 2D NbSe$_2$. Read More

The hybrid halide perovskites, the very performant compounds in photovoltaic applications, possess large Seebeck coefficient and low thermal conductivity making them potentially interesting high figure of merit ($ZT$) materials. For this purpose one needs to tune the electrical conductivity of these semiconductors to higher values. We have studied the CH$_3$NH$_3$MI$_3$ (M=Pb,Sn) samples in pristine form showing very low $ZT$ values for both materials; however, photoinduced doping (in M=Pb) and chemical doping (in M=Sn) indicate that, by further doping optimization, $ZT$ can be enhanced toward unity and reach the performance level of the presently most efficient thermoelectric materials. Read More

We study the vibrational, magnetic and transport properties of Few Layer Graphene (FLG) using Raman and electron spin resonance spectroscopy and microwave conductivity measurements. FLG samples were produced using wet chemical exfoliation with different post-processing, namely ultrasound treatment, shear mixing, and magnetic stirring. Raman spectroscopy shows a low intensity D mode which attests a high sample quality. Read More

We report resistivity, thermoelectric power and thermal conductivity of MoS2 single crystals prepared by chemical vapour transport (CVT) method using I2, Br2 and TeCl4 as transport agents. The material presents low-lying donor and acceptor levels, which dominate the in-plane charge transport. Intercalates into the Van der Waals gap strongly influence the inter-plane resistivity. Read More

We study the Brownian motion of microbeads immersed in water and in a viscoelastic wormlike micelles solution by optical trapping interferometry and diffusing wave spectroscopy. Through the mean-square displacement obtained from both techniques, we deduce the mechanical properties of the fluids at high frequencies by explicitly accounting for inertia effects of the particle and the surrounding fluid at short time scales. For wormlike micelle solutions, we recover the 3/4 scaling exponent for the loss modulus over two decades in frequency as predicted by the theory for semiflexible polymers. Read More

We report on the temperature dependence of thermal conductivity of single crystalline and polycrystalline organometallic perovskite CH3NH3PbI3. The comparable absolute values and temperature dependence of the two sample's morphologies indicate the minor role of the grain boundaries on the heat transport. Theoretical modelling demonstrates the importance of the resonant scattering in both specimens. Read More

Monod and Beuneu [Monod and Beuneu, Phys. Rev. B 19, 911 (1979)] established the validity of the Elliott-Yafet theory for elemental metals through correlating the experimental electron spin resonance line-width with the so-called spin-orbit admixture coefficients and the momentum-relaxation theory. Read More

The synthesis, structural and physical properties of iron lanthanide oxypnictide superconductors, L4Fe2As2Te1-xO4 (L = Pr, Sm, Gd), with transition temperature at ~ 25 K are reported. Single crystals have been grown at high pressure using cubic anvil technique. The crystal structure consists of layers of L2O2 tetrahedra separated by alternating layers of chains of Te and of Fe2As2 tetrahedra: -L2O2-Te-L2O2-Fe2As2-L2O2-Te-L2O2- (space group: I4/mmm, a ~ 4. Read More

To identify possible spin texture contributions to thermoelectric transport, we present a detailed temperature and pressure dependence of thermopower $S$ in MnSi, as well as a low-temperature study of $S$ in a magnetic field. We find that $S/T$ reconstructs the $(p,T)$ phase diagram of MnSi encompassing the Fermi liquid, partially ordered, and non-Fermi liquid phases. Our results indicate that the latter two phases have essentially the same nature. Read More

We have characterized the electrical conductivity of the composite which consists of multi-walled carbon nanotubes dispersed in SU8 epoxy resin. Depending on the processing conditions of the epoxy (ranging from non-polymerized to cross-linked) we obtained tunneling and percolating-like regimes of the electrical conductivity of the composites. We interpret the observed qualitative change of the conductivity behavior in terms of reduced separation between the nanotubes induced by polymerization of the epoxy matrix. Read More

We propose a theoretical model which relies on the generalized Langevin equation and may account for various dynamical features of the thermal motion of organelles, vesicles or macromolecules in viscoelastic media such as polymer networks. In particular, we consider inertial and hydrodynamic effects at short times, subdiffusive scaling at intermediate times, and eventually optical trapping at long times. Simple analytical formulas for the mean square displacement and velocity auto-correlation function are derived. Read More

We observe the electron spin resonance of conduction electrons in boron doped (6400 ppm) superconducting diamond (Tc =3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent ESR signal intensity and its magnitude which is in good agreement with that expected from the density of states through the Pauli spin-susceptibility. Read More

Oxygen vacancies created in anatase TiO2 by UV photons (80 - 130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material. Read More

We report a novel synthesis route of homogeneously manganese-doped titanium dioxide nanotubes in a broad concentration range. The scroll-type trititanate (H(2)Ti(3)O(7)) nanotubes prepared by hydrothermal synthesis were used as precursors. Mn2+ ions were introduced by an ion exchange method resulting Mn(x)H(2-x)Ti(3)O(7). Read More

Field-induced antiferromagnetic (AF) fluctuations and magnetization are observed above the (zero-field) ordering temperature, T_N = 23 K by electron spin resonance in kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl, a quasi two-dimensional antiferromagnet with a large isotropic Heisenberg exchange interaction. The Dzyaloshinskii-Moriya (DM) interaction is the main source of anisotropy, the exchange anisotropy and the interlayer coupling are very weak. The AF magnetization is induced by magnetic fields perpendicular to the DM vector; parallel fields have no effect. Read More

We study the detailed temperature and composition dependence of the resistivity, $\rho(T)$, and thermopower, $S(T)$, for a series of layered bismuth chalcogenides Bi$_2$Te$_{3-x}$Se$_x$, and report the stoichiometry dependence of the optical band gap. In the resistivity of the most compensated member, Bi$_2$Te$_{2.1}$Se$_{0. Read More

We present the investigation of a monoclinic compound SeCuO3 using x-ray powder diffraction, magnetization, torque and electron-spin-resonance (ESR). Structurally based analysis suggests that SeCuO3 can be considered as a 3D network of tetramers. The values of intra-tetramer exchange interactions are extracted from the temperature dependence of the susceptibility and amount to ~200 K. Read More

We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. Read More

Temperature dependent electron spin resonance (ESR) measurements are reported on stage 1 potassium doped graphite, a model system of biased graphene. The ESR linewidth is nearly isotropic and although the g-factor has a sizeable anisotropy, its majority is shown to arise due to macroscopic magnetization. Albeit the homogeneous ESR linewidth shows an unusual, non-linear temperature dependence, it appears to be proportional to the resistivity which is a quadratic function of the temperature. Read More

Polarization dependent vanadium L edge X-ray absorption spectra of BaVS$_3$ single crystals are measured in the four phases of the compound. The difference between signals with the polarization \textbf{E}$\perp$\textbf{c} and \textbf{E}$\parallel$\textbf{c} (linear dichroism) changes with temperature. Besides increasing intensity of one of the maxima, a new structure appears in the pre-edge region below the metal-insulator transition. Read More

Electron spin resonance (ESR) spectroscopy is an important tool to characterize the ground state of conduction electrons and to measure their spin-relaxation times. Observing ESR of the itinerant electrons is thus of great importance in graphene and in single-wall carbon nanotubes (SWCNTs). Often, the identification of CESR signal is based on two facts: the apparent asymmetry of the ESR signal (known as a Dysonian lineshape) and on the temperature independence of the ESR signal intensity. Read More

Observation of the Brownian motion of a small probe interacting with its environment is one of the main strategies to characterize soft matter. Essentially two counteracting forces govern the motion of the Brownian particle. First, the particle is driven by the rapid collisions with the surrounding solvent molecules, referred to as thermal noise. Read More

We report the temperature dependence of the resistivity and thermoelectric power under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. Read More

This is the first study of the effect of pressure on transition metal dichalcogenides intercalated by atoms that order magnetically. Co$_{1/3}$NbS$_2$ is a layered system where the intercalated Co atoms order antiferromagnetically at T$_N$ = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity ($\rho$), thermoelectric power (S) and thermal conductivity ($\kappa$). Read More

We present a multi-frequency Electron Spin Resonance (ESR) study in the range of 4 GHz to 420 GHz of the quasi-one-dimensional, non-dimerized, quarter-filled Mott insulators, delta-(EDT-TTF-CONMe_2)_2X (X=AsF_6, Br). In the high temperature orthorhombic phase above T~190 K, the magnitude and the temperature dependence of the high temperature spin susceptibility are described by a S = 1/2 Heisenberg antiferromagnetic chain with J_AsF6=298 K and J_Br=474 K coupling constants for X=AsF_6 and Br respectively. We estimate from the temperature dependence of the line width an exchange anisotropy, J'/J of ~2 * 10^{-3}. Read More

The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS_2 and TbTe_3 is investigated by means of a multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe remarkable anharmonic effects hitherto undetected in the systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. Read More

A high frequency (111.2-420 GHz) electron spin resonance study of the inter-layer (perpendicular) spin diffusion as a function of pressure and temperature is presented in the conducting phases of the layered organic compounds, {\kappa}-(BEDT-TTF)2-Cu[N(CN)2]X ({\kappa}-ET2-X), X=Cl or Br. The resolved ESR lines of adjacent layers at high temperatures and high frequencies allows for the determination of the inter-layer cross spin relaxation time, Tx and the intrinsic spin relaxation time, T2 of single layers. Read More

We present an ESR study at excitation frequencies of 9.4 GHz and 222.4 GHz of powders and single crystals of a Prussian Blue analogue (PBA), RbMn[Fe(CN)6]*H2O in which Fe and Mn undergoes a charge transfer transition between 175 and 300 K. Read More