arXiv subjects

L. Forró

Publications and source records attributed to L. Forró.

At least 37 records · Page 2Linked to original sources

arXiv2016

Doped carbon nanotubes as a model system of biased graphene

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. We argue that the carbon nanotube ensemble essentially maps out the whole Brillouin zone of graphene thus it acts as a model system of biased graphene.

cond-mat.str-el↗

arXiv2016

Photodiode Response in a CH$_3$NH$_3$PbI$_3$/CH$_3$NH$_3$SnI$_3$ Heterojunction

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$_3$. In the dark state the device behaves as a diode, with the Sn homologue acting as the "p" side. The junction is extremely sensitive to illumination. A complete reversal of the diode polarity, the first observation of its kind, is seen when the junction is exposed to red laser light of 25 mW/cm$^2$ or larger power density. This finding opens up the possibility for a novel class of opto-electronic devices.

cond-mat.mtrl-sci↗

arXiv2016

Metallicity and conductivity crossover in white light illuminated CH$_3$NH$_3$PbI$_3$ perovskite

The intrinsic d.c. electrical resistivity ($ρ$) - measurable on single crystals only - is often the quantity first revealing the properties of a given material. In the case of CH$_3$NH$_3$PbI$_3$ perovskite measuring $ρ$ under white light illumination provides insight into the coexistence of extended and shallow localized states (0.1 eV below the conduction band). The former ones dominate the electrical conduction while the latter, coming from neutral defects, serve as a long-lifetime charge carrier reservoir accessible for charge transport by thermal excitation. Remarkably, in the best crystals the electrical resistivity shows a metallic behaviour under illumination up to room temperature, giving a new dimension to the material in basic physical studies.

cond-mat.str-el↗

arXiv2013

L4Fe2As2Te1-xO4-yFy (L = Pr, Sm, Gd): a layered oxypnictide superconductor with Tc up to 45 K

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 ~ 4.0, c ~ 29.6 Å). Substitution of oxygen by fluorine increases the critical temperature, e.g. in Gd4Fe2As2Te1-xOyF4-y up to 45 K. Magnetic torque measurements reveal an anisotropy of the penetration depths of ~31.

cond-mat.supr-con↗

arXiv2013

Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2

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. The low-temperature enhancement of S/T in the Co substitution range 0.02 < x < 0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and non-reconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05, close to the highest superconducting T_c. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.

cond-mat.supr-con↗

arXiv2011

BaVS$_3$ probed by V L edge X-ray absorption spectroscopy

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. More careful examination brings to light that the changes start already with pretransitional charge density wave fluctuations. Simple symmetry analysis suggests that the effect is related to rearrangements in $E_{g}$ and $A_{1g}$ states, and is compatible with the formation of four inequivalent V sites along the V-S chain.

cond-mat.str-el↗

arXiv2011

Pressure effects on the transport coefficients of Ba(Fe1-xCox)2As2

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. We observe a hole-like contribution to the thermopower below the structural-magnetic transition in the parent compound that is suppressed in magnitude and temperature with pressure. Pressure increases the contribution of electrons to transport in both the doped and undoped compound. In the 10% Co-doped sample, we used a two-band model for thermopower to estimate the carrier concentrations and determine the effect of pressure on the band structure.

cond-mat.supr-con↗

arXiv2010

High pressure study of transport properties in Co$_{1/3}$NbS$_2$

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 ($ρ$), thermoelectric power (S) and thermal conductivity ($κ$). We found that at ambient pressure the magnetic transition corresponds to a well pronounced peak in dS/dT, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to $κ$. Under pressure, the resistivity increases in the high temperature range, contrary to all previous measurements in other layered transition metal dichalcogenides (TMD). In the low temperature range, the strong dependences of thermopower and resistivity on pressure are observed below TN, which, in turn, also depends on pressure at rate of dT$_N$/dp $\approx$ -1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.

cond-mat.str-el↗

arXiv2010

Nonlinear order parameter oscillations and lattice coupling in strongly-driven charge-density-wave systems

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. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent to the broken symmetry state of the CDW systems.

cond-mat.str-el↗

arXiv2010

Inter-layer spin diffusion and electric conductivity in the organic conductors κ-ET2-Cl and κ-ET2-Br

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, κ-(BEDT-TTF)2-Cu[N(CN)2]X (κ-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. In the bad metal phase spin diffusion is two-dimensional, i.e. spins are not hopping to adjacent layers within T2. Tx is proportional to the perpendicular resistivity at least approximately, as predicted in models where spin and charge excitations are tied together. In κ-ET2-Cl, at zero pressure Tx increases as the bad metal-insulator transition is approached. On the other hand, Tx decreases as the normal metal and superconducting phases are approached with increasing pressure and/or decreasing temperature.

cond-mat.str-el↗

arXiv2010

Origin of the ESR spectrum in the Prussian Blue analogue RbMn[Fe(CN)6]*H2O

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. The ESR of PBA powders, also reported by Pregelj et al. (JMMM, 316, E680 (2007)) is assigned to cubic magnetic clusters of Mn2+ ions surrounding Fe(CN)6 vacancies. The clusters are well isolated from the bulk and are superparamagnetic below 50 K. In single crystals various defects with lower symmetry are also observed. Spin-lattice relaxation broadens the bulk ESR beyond observability. This strong spin relaxation is unexpected above the charge transfer transition and is attributed to a mixing of the Mn3+ - Fe2+ state into the prevalent Mn2+ - Fe3+ state.

cond-mat.str-el↗

arXiv2010

Occurrence of superconductivity when the metal-insulator transition is inhibited in $1T$-TaS${_2}$

When a Mott metal-insulator transition is inhibited by a small amount of disorder in the layered dichalcogenide 1T-TaS$_2$, an inhomogeneous superconducting state arises below T=2.1 K, and coexists with a nearly-commensurate charge-density-wave. By angle-resolved photoelectron spectroscopy (ARPES) we show that it emerges from a bad metal state with strongly damped quasiparticles. Superconductivity is almost entirely suppressed by an external magnetic field of 0.1 T.

cond-mat.str-el↗

arXiv2008

Spontaneous exciton condensation in 1T-TiSe2: a BCS-like approach

Recently strong evidence has been found in favor of a BCS-like condensation of excitons in 1\textit{T}-TiSe$_2$. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the excitonic condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. Here, the calculations in the framework of this model are presented in detail. They represent an extension of the original excitonic insulator phase model of Jérome \textit{et al.} [Phys. Rev. {\bf 158}, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and further comparison with experiment are also discussed

cond-mat.str-el↗

arXiv2007

The evolution of the Non-Fermi Liquid behavior of BaVS$_3$ under high pressure

Temperature, pressure, and magnetic field dependencies of the resistivity of BaVS$_3$ were measured above the critical pressure of $p_{cr}$=2 GPa, which is associated with the zero temperature insulator-to-metal (MI) transition. The resistivity exhibits the $T^n$ temperature dependence below $T_g\approx$15 K, with $n$ of 1.5 at $p_{cr}$, which increases continuously with pressure towards 2. This is interpreted as a crossover from non-Fermi (NFL) to Fermi-liquid (FL) behavior. Although the spin configuration of the $e_g$ electrons influences the charge propagation, the NFL behavior is attributed to the pseudogap that appears in the single particle spectrum of the $d_z^2$ electrons related to large quasi-one dimensional (Q-1d) 2$k_F$-CDW fluctuations. The non-monotonic magnetic field dependence of $Δ$$ρ$/$ρ$ reveals a characteristic field $B_0\approx$12 T attributed to the full suppression of the pseudogap.

cond-mat.str-el↗

arXiv2007

Hysteretic behavior at the collapse of the metal-insulator transition in BaVS$_3$

Electrical resistivity as a function of temperature, pressure, and magnetic field was measured in high and low purity single crystals of BaVS$_3$ close to the critical pressure value $p_{cr}$$\approx$2 GPa, associated with the zero temperature insulator-to-metal (MI) transition. In the 1.8-2.0 GPa range, where the MI transition is below $\approx$20 K, one can observe a sudden collapse of the MI phase boundary upon increasing pressure and at fixed pressure a magnetic field induced insulator to metal transition. In high quality samples these features are accompanied by hysteresis in all measured physical quantities as a function of temperature and magnetic field. We ascribe these observations to the crossing of the MI and the magnetic phase boundaries upon increasing pressure.

cond-mat.str-el↗

arXiv2007

Conduction electron spin-lattice relaxation time in the MgB2 superconductor

The spin-lattice relaxation time, $T_{1}$, of conduction electrons is measured as a function of temperature and magnetic field in MgB$_2$. The method is based on the detection of the $z$ component of the conduction electron magnetization under electron spin resonance conditions with amplitude modulated microwave excitation. Measurement of $T_{1}$ below $T_c$ at 0.32 T allows to disentangle contributions from the two Fermi surfaces of MgB$_{2}$ as this field restores normal state on the Fermi surface part with $π$ symmetry only.

cond-mat.supr-con↗

arXiv2006

Quantum critical phase in BaVS$_3$

We study the high-pressure metallic phase of high-purity single crystals of BaVS$_3$ by measuring the temperature, pressure, and magnetic field dependence of the resistivity. Above the critical pressure of $p_{\rm cr}=1.97$GPa an extended non-Fermi liquid $p-T$ regime emerges with resistivity exponent $1.5 \le n<2$, crossing over to a FL only around $p=2.7$GPa. A hysteretic feature indicates that close to the insulator--metal boundary, the system is magnetically ordered. Our findings reveal a close analogy between the extended partially ordered NFL state of non-conventional itinerant magnets and the corresponding state of BaVS$_3

cond-mat.str-el↗

arXiv2006

Spin-lattice interaction in the quasi one-dimensional helimagnet LiCu_2O_2

The field dependence of the electron spin resonance in a helimagnet, LiCu_2O_2, was investigated for the first time. In the paramagnetic state a broad resonance line was observed corresponding to a g-factor of 2.3. In the critical regime around the paramagnetic to helimagnetic phase transition the resonance broadens and shifts to higher frequencies. A narrow signal is recovered at low temperature, corresponding to a spin gap of 1.5 meV in zero field. A comprehensive model of the magnons is presented, using exchange parameters from neutron scattering (T. Masuda et al, Phys. Rev. B 72, 014405), and the spin anisotropy determined here. The role of the quantum fluctuations is discussed.

cond-mat.str-el↗